I haven't played Pokémon Go recently, so I'm assuming you mean the Mewtwo has Psystrike as its charge move. The best counters for that would be Tyranitar with Bite and Crunch, A Giratina with Shadow Claw and Shadow Ball, or a Weavile with Feint Attack and Foul Play. More common Pokémon like Gengar with all ghost moves or Absol with Snarl and Dark Pulse are also good against Mewtwo. Here are some more Pokémon that are good but not quite as good the ones I listed above: Alakazam, Drifblm, Sharpedo, Lucario, Scizor, Banette, and Yanmega, all with super effective moves. Pinsir is a good choice if you don't have many of the others.