About Mega Charizard X's analysis:
Earthquake & Thunderpunch have the same power (thanks to Tough Claws), so Earthquake is also a 3HKO. It makes it sound like Thunderpunch is too weak for Toxapex despite being the same power as Earthquake.
Flare Blitz is an insanely strong STAB move that nukes even Pokemon that resist it. Dragon Claw hits Dragon-types that resist Flare Blitz such as Latios, Zygarde, and Mega Latias and is a more reliable STAB move overall. Earthquake hits Heatran, Toxapex, and Tyranitar, which are otherwise annoying for this set to deal with. Roost allows Mega Charizard X to utilize its rare defensive typing to the fullest and heal off recoil damage sustained from Flare Blitz. Thunder Punch is an option over Earthquake to hit Toxapex, Alomomola, and Tapu Fini in one moveslot, but it makes Charizard struggle against Heatran and Tyranitar. Also, Thunder Punch requires Electric Terrain support from Tapu Koko, as the move fails to 2HKO Toxapex otherwise.