Overview
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Although it might seem out of place in a metagame filled with powerful Psychic- and Ground-types, Poison Arceus shines because of its unique attributes. Unlike most Arceus formes, Poison Arceus is not affected by Toxic and absorbs Toxic Spikes, making it quite difficult for balanced or defensively inclined teams to break. Its Poison typing brings useful resistances to Fairy and Fighting and neutrality to many common attack types, allowing Poison Arceus to easily set up Calm Mind or find turns to support its team with Defog. The typing also enables this particular Arceus forme to serve as a very solid check to the ubiquitous Xerneas, which no other Arceus forme can claim to do other than the risky Fire Arceus. However, Poison Arceus is still somewhat limited by the relative mediocrity of its STAB as an offensive type, and it brings weaknesses to common Ground- and Psychic-type attacks. Although the former can be overcome with Will-O-Wisp and raw bulk, the Psychic weakness makes Poison Arceus horrible bait for extremely threatening Pokemon such as Mewtwo. That being said, Poison Arceus's weaknesses are not too hard to cover, and its useful qualities make it relatively easy to fit onto teams.
Calm Mind
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name: Calm Mind
move 1: Calm Mind
move 2: Judgment / Sludge Bomb
move 3: Will-O-Wisp / Fire Blast
move 4: Recover
item: Toxic Plate
evs: 248 HP / 84 SpA / 176 Spe
nature: Timid
Moves
========
Poison Arceus's inability to be put on a timer by Toxic makes it a very threatening user of Calm Mind. Judgment is generally the preferred STAB option for its higher Base Power. Sludge Bomb is slightly weaker, but carries a cool 30% poison rate that makes wearing down bulkier targets such as Ho-Oh slightly easier. Will-O-Wisp allows Poison Arceus to cripple Ground-types such as Groudon and Landorus-T and continue to set up against them, easily tanking weakened Earthquakes with its solid bulk. Will-O-Wisp also wears down Steel-types such as Klefki and Dialga that are immune to Poison-type attacks. Note that you should never use Sludge Bomb alongside Will-O-Wisp, lest Poison Arceus accidentally poisons a target such as Groudon that it would much rather burn. If you prefer a more proactive approach to dealing with Steel-types, Fire Blast easily cooks most of them save the uncommon Heatran. Fire Blast is still capable of hurting Dialga with a Calm Mind boost, and it deals decent damage to Groudon thanks to Drought. However, Will-O-Wisp is generally more useful, enabling Poison Arceus to set up on Ground-types and giving it utility in checking powerful physical threats such as Mega Lucario and Mega Kangaskhan. Recover rounds out the set by providing Poison Arceus with reliable recovery as it boosts.
Set Details
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The Speed EVs allow Poison Arceus to outspeed Gengar before it Mega Evolves and hit it with Will-O-Wisp or Fire Blast before it has a chance to use Taunt or Destiny Bond. 84 Special Attack EVs allow Poison Arceus to always 2HKO Mewtwo with Judgment after Stealth Rock damage, even if Mewtwo Mega Evolves into Mega Mewtwo Y. The rest of the EVs go into HP to help Poison Arceus take hits as it sets up.
That being said, there are plenty of ways to EV Poison Arceus. Fully investing in Speed to tie with other Calm Mind Arceus formes is possible. The Special Attack EVs can also be invested into Defense to take Earthquake and Mewtwo's Psystrike more effectively, or into Special Defense to give Poison Arceus a better chance of successfully setting up Calm Mind against Geomancy Xerneas.
Usage Tips
========
Poison Arceus is a flexible Pokemon and can be played aggressively to take advantage of its Toxic immunity and solid bulk in order to spread burns and potentially set up. Throwing out Will-O-Wisp aggressively to try to catch Mega Gengar or Ground-types isn't a bad idea. That being said, be very careful with Poison Arceus if the opponent has Mewtwo as it will easily stop Poison Arceus's sweep. Double switching with Poison Arceus and liberally using Judgment are effective ways to deny Mewtwo free turns. It should also be noted that Poison Arceus is effective Xerneas check; just be wary of Psyshock!
Team Options
========
Mewtwo is the biggest threat to Poison Arceus as it is faster and easily OHKOes with Psystrike regardless of how many Calm Mind boosts Poison Arceus has, so having a good check to it is mandatory. Yveltal fills this role superbly, boasting an immunity to Psychic-type attacks and easily wrecking Mewtwo with Sucker Punch or with the aid of Choice Scarf. A physically defensive Yveltal can also aid in checking Mega Mewtwo X. Yveltal is also immune to Ground-type attacks, is good at pressuring tanks such as Ho-Oh and defensive Kyogre, and appreciates Poison Arceus's ability to check Fairy-types, making for an incredibly synergistic pair. Klefki is also a decent Mewtwo check, able to take a Psystrike and cripple Mewtwo with Prankster Thunder Wave. It can also set up Spikes to make Poison Arceus's sweep easier, and it can work as a backup Xerneas check in case Xerneas has Psyshock. If you are especially paranoid about Psyshock Xerneas, Scizor is a good backup check to finish it off after Poison Arceus hits it once, but it might not be necessary to go to such lengths if your team can apply sufficient offensive pressure to Xerneas. Scizor can also revenge kill a weakened Mewtwo with Bullet Punch. Victini can take most of Mewtwo's attacks and destroy it with V-create, and as a bonus it is great at pressuring Steel-type Pokemon. Choice Scarf users such as Kyogre can also work as reasonable Mewtwo checks.
Speaking of Steel-types, they can be annoying to Poison Arceus if it lacks Fire Blast because it has no way to quickly KO them. In the same vein, Ground-types can also be annoying if Poison Arceus lacks Will-O-Wisp. Specially defensive Pokemon such as Kyogre and Ho-Oh are also difficult for Poison Arceus to break through. Landorus-T walls many Ground-type Pokemon and easily smashes Steel-types with its powerful Earthquake. It can also set up Stealth Rock to limit Ho-Oh's ability to switch into Poison Arceus. Palkia and Kyogre are also decent checks to both Ground- and Steel-type Pokemon, most notably Heatran which walls Poison Arceus regardless of whether it carries Will-O-Wisp or Fire Blast. Palkia is particularly useful as it is also an extremely solid check to Kyogre. Zekrom is good at pressuring Kyogre and Ho-Oh, and it can also lure in and cripple Ground-types with a surprise Draco Meteor. Zekrom shares a weakness to Ground-type attacks with Poison Arceus, however, so watch out.
Although Poison Arceus is immune to Toxic, burns can still be annoying and paralysis can grind its sweep to a halt. As such, you might still want support from a cleric. Physically defensive Xerneas is a great cleric that can also serve as a decent Mewtwo check, able to completely wall Mega Mewtwo X and take a hit or two from Mega Mewtwo Y and retaliate with a strong Moonblast.
Defog Support
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name: Defog Support
move 1: Defog
move 2: Poison Jab
move 3: Thunder Wave / Will-O-Wisp
move 4: Recover
item: Toxic Plate
evs: 252 HP / 200 Def / 56 Spe
nature: Impish
Moves
========
One of the most common ways of pressuring support Arceus formes is to smack them with Toxic to ruin their ability to wall threats. However, Poison Arceus is immune to this move, which gives it much more longevity against more balanced teams. This, combined with its ability to act as an excellent Xerneas check, makes Poison Arceus an exceptional Defog user that is particularly dangerous to teams reliant on entry hazards. Poison Jab is preferred to Judgment due to ignoring the Special Defense boost from Geomancy. It 2HKOes all variants of Xerneas easily and wrecks frailer Fairy-types such as Sylveon. It also 3HKOes Gothitelle, preventing it from trapping Poison Arceus. Thunder Wave is the preferred status option as it cripples Mewtwo switch-ins and renders Xerneas unable to sweep regardless of whether or not it is in KO range for Poison Jab. That being said, Will-O-Wisp is still plenty viable for crippling Ground-types and Pokemon such as Klefki, Dialga, and Mega Lucario. Using Thunder Wave renders Poison Arceus vulnerable to Ground-types and unable to stall out Dialga and Klefki by itself whereas using Will-O-Wisp makes it setup fodder for Mewtwo, so carefully consider the needs of your team before picking which status option to use. Recover is invaluable for keeping Poison Arceus alive for as long as it is needed.
Set Details
========
56 Speed EVs are just enough to outrun Adamant Rayquaza; the rest of the EVs go into HP and Defense with an Impish nature to take a boosted Psyshock from Xerneas as well as possible. The heavy investment in physical bulk also makes Poison Arceus a solid check to other physical threats such as Blaziken and Rayquaza, and it can take a Psystrike from Mewtwo if it needs to paralyze Mewtwo or use Defog. Because this set includes Thunder Wave, heavy Speed investment is not required.
However, there is another, equally viable EV spread if using Will-O-Wisp: 252 HP / 160 Def / 96 Spe, with a Jolly nature. This allows Poison Arceus to outspeed Mega Kangaskhan and Palkia and use Will-O-Wisp before they can retaliate. This EV spread is still serviceable for checking Geomancy Xerneas, as it too allows Poison Arceus to survive a boosted Psyshock after Stealth Rock damage. If you have a backup Xerneas check such as Scizor or Klefki on your team, then this EV spread is highly preferred over the listed spread.
Usage Tips
========
This variant of Poison Arceus is an extremely reliable Defog user that can double as a reliable Xerneas check, so it should be used on balanced teams in need of a Pokemon that can do both. The Toxic immunity and ability to absorb Toxic Spikes also makes Poison Arceus very difficult for stall teams to KO and lessens the need for a cleric on balanced teams, which can help free up teambuilding options. Xerneas has a hard time breaking through Poison Arceus, failing to OHKO with Psyshock and only having a low chance to 2HKO with Thunder while it gets crippled by Thunder Wave and smashed by Poison Jab. If it runs Will-O-Wisp, Poison Arceus can defeat Klefki and Dialga one-on-one without them being able to set up entry hazards, something no other Defog user can do. However, taking a turn to use Defog can give something like Mewtwo a free turn to come in, so try to cripple it with Thunder Wave first, and make sure your team is well equipped to deal with it.
Team Options
========
Flying-types make great partners for this set, covering Poison Arceus's Ground-type weakness (especially if using Thunder Wave instead of Will-O-Wisp) while benefiting from Defog removing Stealth Rock. Yveltal is an excellent partner as it handles Mewtwo well while benefiting highly from Poison Arceus's ability to check Fairy-types. Rayquaza and Ho-Oh are also solid partners that can help break down threatening Ground-types. Lugia benefits hugely from Defog and is an extremely solid check to most Ground-type Pokemon.
Mega Gengar is somewhat problematic for this set due to being able to Taunt Poison Arceus to stop Defog and trap it with Shadow Tag. Pursuit trappers such as Chople Berry Tyranitar and Mega Scizor can make Mega Gengar much less of a threat; Tyranitar checks Ho-Oh, Heatran, and Mewtwo while Scizor can serve as a backup Xerneas check. Mega Scizor can also work as a check to Extreme Killer Arceus, which can be important as Poison Arceus is not a terribly good Extreme Killer check due to its weakness to Earthquake, even if it has Will-O-Wisp.
Other than that, Palkia, Landorus-T, and physically defensive Xerneas work well with this set. As this set fits on more balanced teams, Hippowdon can also be considered as a teammate, providing Stealth Rock to help limit Ho-Oh and serving as a check to Ground- and Steel-type Pokemon. It works similarly to Landorus-T, but it can also check Extreme Killer Arceus thanks to Whirlwind.
Other Options
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Poison Arceus has plenty of other options, but it has a hard time fitting them in. Earth Power wipes out Heatran and overeager Mega Gengar while doing solid damage to most other Steel-types, making it a solid option on the Calm Mind set. Grass Knot is useful for dealing solid damage to Groudon and Kyogre, but otherwise it has limited utility. Toxic is a useful option for crippling Mewtwo and Ho-Oh, and it never misses when used by a Poison-type. However, Toxic does nothing to Steel-types, which is problematic. Ice Beam can be used if you really need to OHKO Landorus-T or Gliscor. Stone Edge is a good option to make Poison Arceus into a decent Ho-Oh lure. Punishment is interesting for stopping Mewtwo and Ghost Arceus from trying to set up with Calm Mind, but staying in on these Pokemon is extremely risky and the move has little other utility. Roar isn't a bad option, allowing Calm Mind sets to win Calm Mind wars against other Arceus formes while all but guaranteeing that Xerneas will not be able to sweep.
Checks & Counters
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**Mewtwo**: Mewtwo is the biggest threat to Poison Arceus by far, outspeeding it and easily KOing with its Psychic-type STAB regardless of how many Calm Mind boosts Poison Arceus has. However, it does need to be wary of Thunder Wave.
**Heatran**: Heatran completely walls any set lacking Earth Power and it shuts down Poison Arceus with Taunt.
**Kyogre**: Specially defensive Kyogre tanks hits from Poison Arceus pretty well and can mess it up with Scald burns and Roar.
**Ho-Oh**: Ho-Oh easily takes most hits from Poison Arceus and destroys it with Sacred Fire or Earthquake.
**Shadow Tag**: Pokemon with Shadow Tag are annoying because they render Poison Arceus incapable of switching out. Mega Gengar is the most threatening with Taunt and a 4x resistance to Poison-type attacks, but that same resistance can potentially screw it out of Destiny Bond if it gets burned. Gothitelle can potentially trap Poison Arceus, but it will either be 3HKOed by Poison Jab or have to win a Calm Mind war, so it is not reliable. Wobbuffet can take Poison Arceus down with Mirror Coat or use Encore to make it setup bait for a sweeper.
**Deoxys-A**: Although it cannot come in on any damaging attack, Deoxys-A is faster than Poison Arceus and easily OHKOes with Psycho Boost.
**Ghost Arceus**: Ghost Arceus can usually defeat Poison Arceus in a Calm Mind war given its own resistance to Poison-type attacks.
**Ground-types**: Groudon, Landorus-T, Excadrill, and Ground Arceus all do a lot of damage to Poison Arceus with STAB Earthquake, but they are more often than not shaky checks due to the threat of Will-O-Wisp. Gliscor is immune to burns once its Toxic Orb activates, but its poor Special Defense and low Attack make it a subpar answer to Poison Arceus.
**Steel-types with Recovery**: Some Pokemon cannot defeat Poison Arceus on their own, but they can stall it for a while or use it as bait to punish with entry hazards. Ferrothorn and Skarmory are notable examples so long as Poison Arceus is not running Fire Blast. Skarmory is particularly annoying due to the threat of Taunt. Mega Scizor can also Roost stall Poison Arceus indefinitely if Poison Arceus lacks a direct attack other than its STAB move, but it cannot hurt it much directly.
**Paralysis**: Paralysis ruins Poison Arceus's sweeping potential and can make it vulnerable to threats it could otherwise check, such as Palkia.
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Although it might seem out of place in a metagame filled with powerful Psychic- and Ground-types, Poison Arceus shines because of its unique attributes. Unlike most Arceus formes, Poison Arceus is not affected by Toxic and absorbs Toxic Spikes, making it quite difficult for balanced or defensively inclined teams to break. Its Poison typing brings useful resistances to Fairy and Fighting and neutrality to many common attack types, allowing Poison Arceus to easily set up Calm Mind or find turns to support its team with Defog. The typing also enables this particular Arceus forme to serve as a very solid check to the ubiquitous Xerneas, which no other Arceus forme can claim to do other than the risky Fire Arceus. However, Poison Arceus is still somewhat limited by the relative mediocrity of its STAB as an offensive type, and it brings weaknesses to common Ground- and Psychic-type attacks. Although the former can be overcome with Will-O-Wisp and raw bulk, the Psychic weakness makes Poison Arceus horrible bait for extremely threatening Pokemon such as Mewtwo. That being said, Poison Arceus's weaknesses are not too hard to cover, and its useful qualities make it relatively easy to fit onto teams.
Calm Mind
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name: Calm Mind
move 1: Calm Mind
move 2: Judgment / Sludge Bomb
move 3: Will-O-Wisp / Fire Blast
move 4: Recover
item: Toxic Plate
evs: 248 HP / 84 SpA / 176 Spe
nature: Timid
Moves
========
Poison Arceus's inability to be put on a timer by Toxic makes it a very threatening user of Calm Mind. Judgment is generally the preferred STAB option for its higher Base Power. Sludge Bomb is slightly weaker, but carries a cool 30% poison rate that makes wearing down bulkier targets such as Ho-Oh slightly easier. Will-O-Wisp allows Poison Arceus to cripple Ground-types such as Groudon and Landorus-T and continue to set up against them, easily tanking weakened Earthquakes with its solid bulk. Will-O-Wisp also wears down Steel-types such as Klefki and Dialga that are immune to Poison-type attacks. Note that you should never use Sludge Bomb alongside Will-O-Wisp, lest Poison Arceus accidentally poisons a target such as Groudon that it would much rather burn. If you prefer a more proactive approach to dealing with Steel-types, Fire Blast easily cooks most of them save the uncommon Heatran. Fire Blast is still capable of hurting Dialga with a Calm Mind boost, and it deals decent damage to Groudon thanks to Drought. However, Will-O-Wisp is generally more useful, enabling Poison Arceus to set up on Ground-types and giving it utility in checking powerful physical threats such as Mega Lucario and Mega Kangaskhan. Recover rounds out the set by providing Poison Arceus with reliable recovery as it boosts.
Set Details
========
The Speed EVs allow Poison Arceus to outspeed Gengar before it Mega Evolves and hit it with Will-O-Wisp or Fire Blast before it has a chance to use Taunt or Destiny Bond. 84 Special Attack EVs allow Poison Arceus to always 2HKO Mewtwo with Judgment after Stealth Rock damage, even if Mewtwo Mega Evolves into Mega Mewtwo Y. The rest of the EVs go into HP to help Poison Arceus take hits as it sets up.
That being said, there are plenty of ways to EV Poison Arceus. Fully investing in Speed to tie with other Calm Mind Arceus formes is possible. The Special Attack EVs can also be invested into Defense to take Earthquake and Mewtwo's Psystrike more effectively, or into Special Defense to give Poison Arceus a better chance of successfully setting up Calm Mind against Geomancy Xerneas.
Usage Tips
========
Poison Arceus is a flexible Pokemon and can be played aggressively to take advantage of its Toxic immunity and solid bulk in order to spread burns and potentially set up. Throwing out Will-O-Wisp aggressively to try to catch Mega Gengar or Ground-types isn't a bad idea. That being said, be very careful with Poison Arceus if the opponent has Mewtwo as it will easily stop Poison Arceus's sweep. Double switching with Poison Arceus and liberally using Judgment are effective ways to deny Mewtwo free turns. It should also be noted that Poison Arceus is effective Xerneas check; just be wary of Psyshock!
Team Options
========
Mewtwo is the biggest threat to Poison Arceus as it is faster and easily OHKOes with Psystrike regardless of how many Calm Mind boosts Poison Arceus has, so having a good check to it is mandatory. Yveltal fills this role superbly, boasting an immunity to Psychic-type attacks and easily wrecking Mewtwo with Sucker Punch or with the aid of Choice Scarf. A physically defensive Yveltal can also aid in checking Mega Mewtwo X. Yveltal is also immune to Ground-type attacks, is good at pressuring tanks such as Ho-Oh and defensive Kyogre, and appreciates Poison Arceus's ability to check Fairy-types, making for an incredibly synergistic pair. Klefki is also a decent Mewtwo check, able to take a Psystrike and cripple Mewtwo with Prankster Thunder Wave. It can also set up Spikes to make Poison Arceus's sweep easier, and it can work as a backup Xerneas check in case Xerneas has Psyshock. If you are especially paranoid about Psyshock Xerneas, Scizor is a good backup check to finish it off after Poison Arceus hits it once, but it might not be necessary to go to such lengths if your team can apply sufficient offensive pressure to Xerneas. Scizor can also revenge kill a weakened Mewtwo with Bullet Punch. Victini can take most of Mewtwo's attacks and destroy it with V-create, and as a bonus it is great at pressuring Steel-type Pokemon. Choice Scarf users such as Kyogre can also work as reasonable Mewtwo checks.
Speaking of Steel-types, they can be annoying to Poison Arceus if it lacks Fire Blast because it has no way to quickly KO them. In the same vein, Ground-types can also be annoying if Poison Arceus lacks Will-O-Wisp. Specially defensive Pokemon such as Kyogre and Ho-Oh are also difficult for Poison Arceus to break through. Landorus-T walls many Ground-type Pokemon and easily smashes Steel-types with its powerful Earthquake. It can also set up Stealth Rock to limit Ho-Oh's ability to switch into Poison Arceus. Palkia and Kyogre are also decent checks to both Ground- and Steel-type Pokemon, most notably Heatran which walls Poison Arceus regardless of whether it carries Will-O-Wisp or Fire Blast. Palkia is particularly useful as it is also an extremely solid check to Kyogre. Zekrom is good at pressuring Kyogre and Ho-Oh, and it can also lure in and cripple Ground-types with a surprise Draco Meteor. Zekrom shares a weakness to Ground-type attacks with Poison Arceus, however, so watch out.
Although Poison Arceus is immune to Toxic, burns can still be annoying and paralysis can grind its sweep to a halt. As such, you might still want support from a cleric. Physically defensive Xerneas is a great cleric that can also serve as a decent Mewtwo check, able to completely wall Mega Mewtwo X and take a hit or two from Mega Mewtwo Y and retaliate with a strong Moonblast.
Defog Support
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name: Defog Support
move 1: Defog
move 2: Poison Jab
move 3: Thunder Wave / Will-O-Wisp
move 4: Recover
item: Toxic Plate
evs: 252 HP / 200 Def / 56 Spe
nature: Impish
Moves
========
One of the most common ways of pressuring support Arceus formes is to smack them with Toxic to ruin their ability to wall threats. However, Poison Arceus is immune to this move, which gives it much more longevity against more balanced teams. This, combined with its ability to act as an excellent Xerneas check, makes Poison Arceus an exceptional Defog user that is particularly dangerous to teams reliant on entry hazards. Poison Jab is preferred to Judgment due to ignoring the Special Defense boost from Geomancy. It 2HKOes all variants of Xerneas easily and wrecks frailer Fairy-types such as Sylveon. It also 3HKOes Gothitelle, preventing it from trapping Poison Arceus. Thunder Wave is the preferred status option as it cripples Mewtwo switch-ins and renders Xerneas unable to sweep regardless of whether or not it is in KO range for Poison Jab. That being said, Will-O-Wisp is still plenty viable for crippling Ground-types and Pokemon such as Klefki, Dialga, and Mega Lucario. Using Thunder Wave renders Poison Arceus vulnerable to Ground-types and unable to stall out Dialga and Klefki by itself whereas using Will-O-Wisp makes it setup fodder for Mewtwo, so carefully consider the needs of your team before picking which status option to use. Recover is invaluable for keeping Poison Arceus alive for as long as it is needed.
Set Details
========
56 Speed EVs are just enough to outrun Adamant Rayquaza; the rest of the EVs go into HP and Defense with an Impish nature to take a boosted Psyshock from Xerneas as well as possible. The heavy investment in physical bulk also makes Poison Arceus a solid check to other physical threats such as Blaziken and Rayquaza, and it can take a Psystrike from Mewtwo if it needs to paralyze Mewtwo or use Defog. Because this set includes Thunder Wave, heavy Speed investment is not required.
However, there is another, equally viable EV spread if using Will-O-Wisp: 252 HP / 160 Def / 96 Spe, with a Jolly nature. This allows Poison Arceus to outspeed Mega Kangaskhan and Palkia and use Will-O-Wisp before they can retaliate. This EV spread is still serviceable for checking Geomancy Xerneas, as it too allows Poison Arceus to survive a boosted Psyshock after Stealth Rock damage. If you have a backup Xerneas check such as Scizor or Klefki on your team, then this EV spread is highly preferred over the listed spread.
Usage Tips
========
This variant of Poison Arceus is an extremely reliable Defog user that can double as a reliable Xerneas check, so it should be used on balanced teams in need of a Pokemon that can do both. The Toxic immunity and ability to absorb Toxic Spikes also makes Poison Arceus very difficult for stall teams to KO and lessens the need for a cleric on balanced teams, which can help free up teambuilding options. Xerneas has a hard time breaking through Poison Arceus, failing to OHKO with Psyshock and only having a low chance to 2HKO with Thunder while it gets crippled by Thunder Wave and smashed by Poison Jab. If it runs Will-O-Wisp, Poison Arceus can defeat Klefki and Dialga one-on-one without them being able to set up entry hazards, something no other Defog user can do. However, taking a turn to use Defog can give something like Mewtwo a free turn to come in, so try to cripple it with Thunder Wave first, and make sure your team is well equipped to deal with it.
Team Options
========
Flying-types make great partners for this set, covering Poison Arceus's Ground-type weakness (especially if using Thunder Wave instead of Will-O-Wisp) while benefiting from Defog removing Stealth Rock. Yveltal is an excellent partner as it handles Mewtwo well while benefiting highly from Poison Arceus's ability to check Fairy-types. Rayquaza and Ho-Oh are also solid partners that can help break down threatening Ground-types. Lugia benefits hugely from Defog and is an extremely solid check to most Ground-type Pokemon.
Mega Gengar is somewhat problematic for this set due to being able to Taunt Poison Arceus to stop Defog and trap it with Shadow Tag. Pursuit trappers such as Chople Berry Tyranitar and Mega Scizor can make Mega Gengar much less of a threat; Tyranitar checks Ho-Oh, Heatran, and Mewtwo while Scizor can serve as a backup Xerneas check. Mega Scizor can also work as a check to Extreme Killer Arceus, which can be important as Poison Arceus is not a terribly good Extreme Killer check due to its weakness to Earthquake, even if it has Will-O-Wisp.
Other than that, Palkia, Landorus-T, and physically defensive Xerneas work well with this set. As this set fits on more balanced teams, Hippowdon can also be considered as a teammate, providing Stealth Rock to help limit Ho-Oh and serving as a check to Ground- and Steel-type Pokemon. It works similarly to Landorus-T, but it can also check Extreme Killer Arceus thanks to Whirlwind.
Other Options
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Poison Arceus has plenty of other options, but it has a hard time fitting them in. Earth Power wipes out Heatran and overeager Mega Gengar while doing solid damage to most other Steel-types, making it a solid option on the Calm Mind set. Grass Knot is useful for dealing solid damage to Groudon and Kyogre, but otherwise it has limited utility. Toxic is a useful option for crippling Mewtwo and Ho-Oh, and it never misses when used by a Poison-type. However, Toxic does nothing to Steel-types, which is problematic. Ice Beam can be used if you really need to OHKO Landorus-T or Gliscor. Stone Edge is a good option to make Poison Arceus into a decent Ho-Oh lure. Punishment is interesting for stopping Mewtwo and Ghost Arceus from trying to set up with Calm Mind, but staying in on these Pokemon is extremely risky and the move has little other utility. Roar isn't a bad option, allowing Calm Mind sets to win Calm Mind wars against other Arceus formes while all but guaranteeing that Xerneas will not be able to sweep.
Checks & Counters
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**Mewtwo**: Mewtwo is the biggest threat to Poison Arceus by far, outspeeding it and easily KOing with its Psychic-type STAB regardless of how many Calm Mind boosts Poison Arceus has. However, it does need to be wary of Thunder Wave.
**Heatran**: Heatran completely walls any set lacking Earth Power and it shuts down Poison Arceus with Taunt.
**Kyogre**: Specially defensive Kyogre tanks hits from Poison Arceus pretty well and can mess it up with Scald burns and Roar.
**Ho-Oh**: Ho-Oh easily takes most hits from Poison Arceus and destroys it with Sacred Fire or Earthquake.
**Shadow Tag**: Pokemon with Shadow Tag are annoying because they render Poison Arceus incapable of switching out. Mega Gengar is the most threatening with Taunt and a 4x resistance to Poison-type attacks, but that same resistance can potentially screw it out of Destiny Bond if it gets burned. Gothitelle can potentially trap Poison Arceus, but it will either be 3HKOed by Poison Jab or have to win a Calm Mind war, so it is not reliable. Wobbuffet can take Poison Arceus down with Mirror Coat or use Encore to make it setup bait for a sweeper.
**Deoxys-A**: Although it cannot come in on any damaging attack, Deoxys-A is faster than Poison Arceus and easily OHKOes with Psycho Boost.
**Ghost Arceus**: Ghost Arceus can usually defeat Poison Arceus in a Calm Mind war given its own resistance to Poison-type attacks.
**Ground-types**: Groudon, Landorus-T, Excadrill, and Ground Arceus all do a lot of damage to Poison Arceus with STAB Earthquake, but they are more often than not shaky checks due to the threat of Will-O-Wisp. Gliscor is immune to burns once its Toxic Orb activates, but its poor Special Defense and low Attack make it a subpar answer to Poison Arceus.
**Steel-types with Recovery**: Some Pokemon cannot defeat Poison Arceus on their own, but they can stall it for a while or use it as bait to punish with entry hazards. Ferrothorn and Skarmory are notable examples so long as Poison Arceus is not running Fire Blast. Skarmory is particularly annoying due to the threat of Taunt. Mega Scizor can also Roost stall Poison Arceus indefinitely if Poison Arceus lacks a direct attack other than its STAB move, but it cannot hurt it much directly.
**Paralysis**: Paralysis ruins Poison Arceus's sweeping potential and can make it vulnerable to threats it could otherwise check, such as Palkia.
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