He never said this is for doubles, so earthquake is still a good move. Mega Charizard X has relatively weak special attack, so heat wave is a bad idea. Dragon tail is weak and is low priority, so the opponent gets a chance to move before getting hit. Dig and solar beam only serve to give opponents a free turn to switch into a Pokemon that resists those attacks. They can also use that turn to use a status move like stealth rock, dragon dance, or recover.