Dream Eater is wasted on Gengar - He has a super devastating Special Attack.
My Gengar goes in for the Kill and splits when need be:
Shadow Ball (devastating, even when hitting for neutral damage, it will dent a lot of foes)
Focus Blast (iffy acuracy, but still worth the risk, for the normals that would otherwise take advantage)
Pain Split (to be used wisely)
Last slot can be up to you, Hypnosis is probably best, but substitute also works, and you could even use explosion for when Gengar is on his last legs
But forget Dream eater - use him for what he does best; a killer special attacker!