252 SpA Mega Charizard Y Fire Blast vs. 0 HP / 0 SpD Mewtwo in Sun: 343-405 (97.1 - 114.7%) -- 81.3% chance to OHKO
Seems like more than "some" damage. Modest could even be run since Charizard won't be targeted if we assume that Blastoise will be hit first, in which case:
252+ SpA Mega Charizard Y Fire Blast vs. 0 HP / 0 SpD Mewtwo in Sun: 375-442 (106.2 - 125.2%) -- guaranteed OHKO