Gengar has a phenomenal Special Attack and decent Speed, plus good moves like Sludge Wave, Shadow Ball, Psychic, and Thunderbolt. It's quite frail, however.
Dragapult is even faster than Gengar, and has an excellent Attack stat. It can go physical, special, or mixed easily, with good moves like Dragon Darts, Shadow Ball, U-turn, and Thunderbolt. It too is quite frail.
I would recommend Gengar, as it has a slight attacking edge, and you don't have to level it up absurdly. You also have an abundance of physical attackers already.
Gengar @ Life Orb
Ability: Cursed Body
- Sludge Wave
- Shadow Ball
- Psychic / Giga Drain
Hope I helped!