Ok, there's 2 Pokemon I see being a major problem and another that can be really annoying under certain circumstances:
Durant @ Choice Band
Ability: Hustle
EVs: 252 Atk / 4 Def / 252 Spe
Jolly Nature
- X-Scissor
- Iron Head
- SuperPower
- Thunder Fang / Baton Pass
Really, any variant of Durant threatens to trash this core really badly, but the Choice Band threat is most threatening since it doesn't take a turn to use Hone Claws and just fires off attacks right off the bat. It fears Blizzard, Focus Blast, and Scald, but these are the only moves this core can threaten Durant with. If Durant gets in, you'd better have a reliable response because there's no way that Mega Abomasnow and Slowking are taking a single X-Scissor. Iron Head and SuperPower also do a hefty amount to Mega Abomasnow, so things are worse if Durant gets in against it.
Escavalier @ Assault Vest / Choice Band
Ability: Overcoat
EVs: 164 HP / 252 Atk / 92 Spe
Adamant Nature
- Megahorn
- Iron Head
- Knock Off
- Drill Run / Pursuit
Escavalier is perhaps even worse for this core as Mega Abomasnow has no real way to deter it from coming in. Slowking can at least threaten a Scald burn and most smart players aren't going to send in Escavalier against Slowking directly in fear of a possible Fire Blast, but Scald is the only move Escavalier fears from this core. What makes matters worse is that Overcoat ensures that Escavalier isn't worn down by Hail damage. Megahorn rips both Mega Abomasnow and Slowking to shreds. Choice Banded Escavaliers aren't as durable vs. Mega Abomasnow, but many of these run Pursuit, which means they will be more of a pain for Slowking to deal with.
Houndoom @ Life Orb
Ability: Flash Fire
EVs: 220 Atk / 36 SAtk / 252 Spe
Hasty Nature
- Sucker Punch
- Pursuit
- Fire Blast
- Crunch
Much like with Durant, any Houndoom set can be a nuisance once Slowking loses its Colbur Berry, but the trapper set is worse thanks to this Houndoom hitting Slowking on its weaker defense stat. Thankfully for this core, Slowking's Colbur Berry can feign Choice Specs, thus allowing you to lure Houndoom in with a Psyshock, only to eat a Crunch / Pursuit and Scald it for a KO. However, if the Colbur Berry has already been used up, Houndoom can become quite tricky for this core to work around, although Mega Abomasnow's sheer power is enough to prevent free switch-ins unless it's off of a KO.