This is what I use:
Gengar (M) @ Focus Sash
252 SP.ATK/252 SPEED/4 SP.DEF
Venoshock (One of the most underrated moves in the entire game with over 180 base power when the opponent is poisoned and with STAB)
The Basic strategy here is Toxic something, Venoshock it, Shadow Ball it if it survives, rinse and repeat until you're on low HP, then Destiny Bond.If you have a dedicated toxic spiker on your team then set those up and sub toxic out for something like Thunderbolt, Ice Beam or Psychic for better coverage. The bonus to this set is if it looks like you're gonna be OHKO'd first turn, straight away use destiny bond. either you'll outspeed which is the best case scenario, or you'll get knocked down to your focus sash and then use destiny bond which means they can't hit next turn without sacrificing their sweeper.