Dark Pulse. You said you wanted a Sp Attacker. Here's a move set for this Dark Pulse Gengar.
Nature: Modest
Move set:
Dark Pulse (Obvious)
Shadow Ball (STAB, may lower Sp. D)
Sludge Bomb (STAB, may poison)
Psychic (Good power, may lower Sp. D)
Hope this works out for you!