It has a huge special attack stat, several multi-target moves, and can be paired with Storm Drain to eliminate its biggest weakness. Works well with Gastrodon.
It can stop water attacks directed at Camerupt and hit back with several multi-target moves. Grass types can't come in without being threatened by Camerupt.
Works well with Camerupt.
Gengar-Mega (but only if you aren't playing with Smogon clauses)
Perish Song combined with Shadow Tag can trap and KO 2 opponents at once. However, you can't use it on Showdown.