Unknown Dungeon (Kalos)
While the Master Ball is the only surefire way to catch Mewtwo without having to worry about it spamming Recover, there are ways to contain that. Moves such as Taunt and Heal Block will temporarily prevent Mewtwo from healing, giving you a chance to bring its HP down further.
But as you do that, whichever Pokémon that you use will need to be able to take hits. A Bronzong of similar level will quad-resist the move Psychic and take normal damage from Aura Sphere, plus it has access to Heal Block, which will keep it from using Recover. Sableye and Spiritomb wall both of Mewtwo's attacks thanks to a double-typing that is immune to both Fighting- and Psychic-Type moves, and while they won't have Heal Block they can use Taunt effectively.
Lastly, without the Master Ball, bring plenty of Ultra Balls, Dusk Balls and Timer Balls.
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