Garchomp with or without Scarf outspeeds and OHKO Mega-Charizard X with Earthquake, Stone Edge and Outrage.
Weakness Policy Dragonite
Multiscale Weakness Policy Dragonite resists Dragon Claw and gets a boost on the way. After a Dragon Dance, it outspeeds and OHKO Mega-Charizard X with either Earthquake or Dragon Claw.
Haze and Phaze
Actually, there is a pun in this strategy. Dragonite fulfills the role of both if Forretress can setup Stealth Rock and hopefully at least one or two layer of Spikes, if Custap Lead with Sturdy. Dragonite has access to Haze to eliminate Mega-Charizard X's boosts, or it boasts Roar for phazing and Stealth Rock and Spikes will damage Charizard when it comes back.
Bulky and Offensive Water-type Pokémons
Hey, I point at you Mega-Gyarados and Azumarill. RestTalk Gyarados works as a solid check with Intimidate as it is able to phaze Charizard out with Dragon Tail. Azumarill, on a similar note, can come in on any Dragon-type attack, take a hit, and OHKO with a Play Rough after Stealth Rock.
Hippowdon can Slack Off any damage and phaz with Whirlwind.
Landorus can Intimidate Mega-Charizard X and KO it with Earthquake.
Mega-Charizard X is a force to be reckoned with, but if it cannot setup, then it will not OHKO Mega-Aerodactyl who can Taunt and OHKO with Stone Edge.
Heatran is a capable counter to Mega-Charizard X but only if Charizard is locked into one move by the use of Encore, or if Earthquake's been PP Stalled or something. Otherwise, Sash Heatran can retaliate with Earth Power.
I can't think of other counters.
I hope this helps :)
~ Sincerely yours, Blitz of Justice.