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Slurpuff builds, anyone? I have a few.
EVs: 36 HP, 96 Def, 192 SpAtk, 138 Sp.Def, 48 Spd
Ability: Sweet Veil to avoid being put to sleep and dunk'd on
Item: Big Root to give Draining Kiss 100% regen-to-damage rate
Moves: Calm Mind, Cotton Guard, Draining Kiss, Flamethrower/Psychic/Surf (depending on whether you want to cover Steel weakness, Poison weakness, or hit both neutrally; I prefer Flamethrower for the Burn chance)
Lead with Slurpuff, use either Calm Mind or Cotton Guard depending on what enemy you're facing (1 Cotton Guard followed by two Calm Minds or vice versa). Congratulations, you're FUCKING INVINCIBAAAAAALLL. You are now free to continue boosting to your heart's content, using STAB, Big Root-boosted Draining Kiss whenever your health gets low.
Slurpuff eats most common Taunt users, but status effects can ruin it. But I've swept entire teams of legendaries using only this set-up, and you can too.