Gengar

Gengar is a solid Ghost / Poison type Pokemon that can fill many roles, including supporter, sweeper, and Perish Song user. Gengar's unique typing and ability give it immunities to the common Normal-, Fighting-, and Ground-type moves. Because of this, Gengar is also one of the best Mega Kangaskhan counters that you can ask for, as it is immune to most of Mega Kangaskhan's attacks and is able to burn it with Will-O-Wisp. Shadow Tag is able to trap Pokemon, allowing Mega Gengar to throw off your opponent's game plan and put them in dire positions for you to take advantage of. Mega Gengar is also able to snipe specific Pokemon and remove them to pave the way for the rest of your team. However, Gengar is rather frail and cannot be played recklessly.