If you don't care of fighting with annoying people using OP legends, you might use this:
Gengar @ Gengarite
Ability: Cursed Body
EVs: 4 HP / 252 SpA / 252 Spe
Modest Nature
- Will-O-Wisp
- Hex
- Thunderbolt
- Focus Blast
Will-O-Wisp (burns the foe, taking care of it's physical attack. gengar special defense is nice anyways, so it may make it a bit bulkier)
Hex (482 SpA. + 130 BP STAB move = OHKO. don't need to say anything more, eh?)
Thunderbolt (coverage, nice power, almost always hits, plus it can paralize, making it such as even faster)
Focus Blast (Excellent power, not the best accuracy, but shadow tag traps the opponent making it unable to flee even if you miss. and if it avoids shadow tag by being a ghost type, why in the world would you use focus blast?)