Nasty plot and 3 special moves (dark pulse/flamethrower/focus blast) with a Gengar on the back. Sludge bomb and/or shadow ball can be used alternatively to further fuel the illusion that you're actually a Gengar.
Gengar is ghost/poison, so the opponent is unlikely to try and hit you with fairy, fighting or bug moves. They will try to kill it with psychic, dark or ghost, all of which zoroark either resists or is not affected by.
Gengar is usually very strong on its own for the other player to risk trying to foil your ruse if they suspect anything.
Even then, both pokemon share similar atributes, like being special and fast, with the only difference being gengar not learning nasty plot (for whatever fucking reason), so if your opponent knows this the illusion is discovered right away. If they don't, have fun.
