Moveset for Gengar? What do you think?

I was thinking of using Shadow Ball, Thunderbolt, Focus Blast, and Sludge Bomb. Moves I was considering are Explosion and Toxic, and a slip chance of Focus Punch. I don't really want to use Explosion because my Metagross knows it, but if it's a good choice I'd like to be convinced otherwise. Focus Punch is nearly all a luck game, and its a physical move. Sludge Bomb has STAB and does damage even if it doesn't poison, but Toxic can kill a Blissey if it hits (eventually); then again Sludge Bomb would eventually poison/kill Blissey anyways.