Cowtao

To be honest, Florges can take a Sludge Bomb from Mega Gengar... Oh and Mega Gengar actually has a hard time against Mega Mawile...

Genesect > Every Fairy

Mega Gengar is certainly a threat (for the time being anyway, it's going to get banned to Ubers...) but certainly not the bane of their existence... Like Tomas said, Mega Mawile and Florges can take a hit from it, and potentially knock it out if it doesn't have Focus Sash.

Mega Gengar is a Fairy CHECK, not a COUNTER like Genesect with Iron Head and FlameThrower is...

Oh right Gengar can't safely switch into most Fairy types which contributes to my statement of it be a check rather than a counter...