Acid Meltdown
Item: Leftovers/Black Sludge
Nature: Modest
Ability: Liquid Ooze/Rain Dish
EVs: 252 SpAtk/44 SpDef/212 Speed
Moveset:
-Acid Spray
-Giga Drain
-Surf/Ice Beam
-Hidden Power Fire/Ice Beam/Rapid Spin/Knock Off
An all out attacking set for Tentacruel? Is this dude crazy?
Yes, he is. This set is off of the same block as the "Metal Sound" sets Heatran and Zapdos ran in Gen 4.
Acid Spray is main STAB. Better accuracy than Metal Sound, and deals some damage, while lowering SpDef by two stages.
Giga Drain is second moveslot. After the SpDef drop, you have a chance to restore a decent chunk of lost HP, especially if the likes of Chansey/Blissey tries to wall you.
In move slot three, Surf gets the main slash for a hard hitting second STAB, or, if you are either okay running Acid Spray as your only STAB move, Ice Beam can hit Dragon types hard on the switch. Also, Ice Beam should be used if you are using this on a sun team.
Last move slot main slash goes to Hidden Power Fire, so you aren't walled by Steel types. In the sun, this will do some serious damage to Steel types, putting Scizor, Metagross, Ferrothor, Forrettress, Skarmory, and Virision all within OHKO, or 2HKO range, and outspeeding all of them except Virision. If being used in the rain (w/ Rain Dish ability), Ice Beam or Rapid Spin can be used in place of Hidden Power Fire.
The EVs given are to outspeed, and 2HKO Gyarados on the switch, or an unboosted Gyarados assuming Stealth Rocks are on the field. If you are using this in the rain, you can minus four EVs from Speed, and place them into SpDef (for a secondary spread of 252 SpAtk/208 Speed/48 SpDef). The spread outspeeds Heatran and Tyranitar, as well, although you shouldn't stay in on Tyranitar unless it's already weakened quite a bit.
In the sun: Tentacruel can help Ninetails, Venusaur, and company counter common threats, and hit all other weather inducers for super effective damage. Politoed and Abomasnow do laughable damage to this set, as well. Surf is neutered in the Sun. Ice Beam, Rapid Spin, or Knock Off could be used in Surf's usual place. In the sun, use Liquid Ooze ability.
In the rain: After hitting Acid Spray in the rain, Surf will do enormous damage. You can also fend off most of the common Pokes on a sun team, as well. In the rain, use Rain Dish ability, with a spread of 252 SpAtk/208 Speed/48 SpDef.
In clear skies: Balls to the wall hyper offence tactics. Hit your opponent fast and hard. Makes for a great lead in clear skies. An opponent will more often than not Taunt Tentacruel if their lead is carrying Taunt, giving you the upper hand. In clear Skies, use Liquid Ooze ability.