Hmm for megas Charizard Y could be good for doubles. Also what Lux said. maybe mega Kangaskhan?
One I suggest is Greninja with the Protean ability. I've won many matches with thanks to protean and Mat block in doubles and triple battle. the other moves i gave it were Ice beam, scald, and extrasensory.
Protect is a great move you should give your pokemon.
And I guess maybe gengar?
Good luck! I know you'll do great! :)