This is what I did: Have the shadow ponyta and quagsire that you resently caught in the volcano on your team so that Mewtwo's attacks get resisted. Any high leveled shadow pokemon is fine though. Then put ponyta first on the team and attack the mewtwo and pray to the RNG lords that Mewtwo enters hyper mode first turn. After that stall with ponyta and quagsire until they die and have at least 2 or one priority attack move pokemon to get Mewtwo low enough (in my case it was a shuppet I got from a building). In any case, it's all RNG.
