First off, you need to be listening to this while you read my set http://www.youtube.com/watch?v=VEA_hodDXBg&feature=related
It's borderline essential.
Now that we have that out of the way, I'm going to listen to something different while I write this set out :P

Butterful the Butterfree @ Life freakin' orb
Ability: Tinted Lens
Nature: Modest
EVs: 4 HP / 252 SP ATK / 252 SPEED
Quiver Dance - Let the boosting begin. Try to use this when you're pretty darn sure your opponent will switch out. I HIGHLY RECOMMEND using Stealth Rocks as team support before bringing this thing out ( and spinning away any rocks on your side ) so that you can land OHKOs on opponents that are abusing sash. They are out there!
Bug Buzz - STAB. Tinted Lens makes your not very effective hits neutral. That's pretty darn awesome if you ask me. This will be your primary attacking move.
Sleep Powder - Even though I'm not running the set where this is accurate ( the ability I mean.. whatevs ) it's still ridiculously useful for boosting. Taking the risk normally pays off..... normally.
Psychic / Shadow Ball - Choose your coverage. Both work remarkably well. Both hit Gengar / Haunter for super effective damage ( he 4X resists Bug buzz, so he'd still 2X resist you even after Tinted Lens. )