>>15145876Mega Gengar by definition doesn't have a counter.
He is revenged easily, though. Pursuit bait, tanky priority (Scizor, Azumarill, Talonflame, etc..), and scarfed users with SE moves like Garchomp, Hydreigon, or Latios when it's released.
If it gets banned, it won't be due to it's inability to die. It'll be because it's a gauranteed one KO.