I know the set most Mega Gengar use like a Shadow Tagged Perish Song with Substitute....but there are tons of counters for Mega Gengar
For one,Mega Gengar doesn't have Levitate,so he can't avoid Earthquake or Earth Power,not to mention that Gengar and his mega evolution are frail and can barely survive a super effective attack especially things like an Alakazam(without his mega evolution)or Excadrill
For one,Mega Gengar doesn't have Levitate,so he can't avoid Earthquake or Earth Power,not to mention that Gengar and his mega evolution are frail and can barely survive a super effective attack especially things like an Alakazam(without his mega evolution)or Excadrill