Gengar Prime is ok. But its pokebody requires that Gengar be in the active spot. That being said, gengar must rely on its own attacks to KO pokemon and use its body.
Its second attack is crap. Placing 4 damage counters in any way you like.
....Okay, its not a bad attack, but you wont be KOing anything very quickly.
And its other attack, Plunge into Darkness, IS the wonder attack, but there will be ways around it.
Plus Gengar is weak to Dark. Or Absol G Lv X.