Grass types are good ones, Serperior can create a nice offensive core with Zard, Celebi can Support Charizard with Healing Wish and set up Spikes.What are some checks to rain that pair well with Mega Charizard X?
Some water types can pair well also, Manaphy is similar to Serperior, but it takes better advantage from rain, Azumarrill has a great synergy with Zard X, because they deals with each other checks.