Gengar @ Petaya Berry {Spd+ PhAt-} [252+SpAt 252+Spd 6+HP]
Thunderbolt/Psychic
Mean Look
Hypnosis
Nightmare
I would have used Seismic Toss, but it's putting Gengar's high Special Attack to waste. If you want Seismic Toss, go ahead; make sure to readjust the IVs.
Mean Look the tank. Then, use Hypnosis, followed by Nightmare for a constant drain on the tank's HP. Being a Posion-type, Gengar cannot be poisoned. This makes Gengar a good choice, since many tanks rely on Toxic. Of course, I'm biased; just check out my avatar.
Or change it a bit:
Gengar @ Petaya Berry {Spd+ PhAt-} [252+SpAt 252+Spd 6+HP]
Thunderbolt/Psychic
Seismic Toss
Mean Look
Taunt/Mist