Mewtwo was easy. Just paralyze Mewtwo and attack until Mewtwo's health is red. It took five of my turns to accomplish that with a level 53 Zapdos. Then, I used the Master Ball on Mewtwo. Mewtwo was at level 70. It was, however, harder to catch Zapdos, Moltres, and Articuno than to catch Mewtwo. That's because using Ultra Balls on them is hard work, especially when the Master Ball is sitting there like, "Dude, I come with a 'never-fails' guarantee." I waited to use the Master Ball on Mewtwo after my fifth turn because I've seen the Poke Balls, Great Balls, and Ultra Balls fail too many times to trust the 'never fails' line, no matter how tempting.