Mewtwo is one of those legendary Pokémon that doesn't evolve in the traditional sense—it's already at its peak! But if you're looking for ways to 'power up' Mewtwo, you might be thinking of Mega Evolution or its Gigantamax form in certain games. In 'Pokémon X' and 'Y', as well as 'Omega Ruby' and 'Alpha Sapphire', you can mega evolve Mewtwo using either Mewtwonite X or Y. You'll need to equip the right stone and have a strong bond with your Mewtwo to trigger the transformation mid-battle.
Gigantamax Mewtwo is a bit different—it's only available in 'Pokémon Sword' and 'Shield' during special raid events. Honestly, catching and training Mewtwo is already a feat, so getting to mega evolve or Gigantamax it feels like the ultimate flex. I remember spending hours grinding for the right stones, and the payoff was totally worth it when my Mewtwo transformed mid-battle and wiped the floor with my rival.