Yes, about 9 times in 10. Four things do it:
• Sturdy: from Pokémon Black and White on, a Pokémon with Sturdy that is at full HP survives any single hit with 1 HP.
• Berry Juice: a held item that heals 20 HP when HP drops to half or less. A level 1 Magnemite has only 11 or 12 HP, so that is a full heal, and Sturdy works again.
• Recycle: brings the used Berry Juice back, and Magnemite drinks it again at once.
• Toxic: badly poisoned. The damage grows every turn: 1/16 of Mewtwo’s HP, then 2/16, 3/16 and so on. After six turns that adds up to more than all of its HP.
Checked in 10,000 simulated battles with the rules of Pokémon Ultra Sun and Ultra Moon: Magnemite wins 9 times in 10 against a Mewtwo that keeps attacking with Psystrike, always on turn six. It loses when Toxic misses (90% accuracy). A burn, a freeze, hail or Taunt also break the trick.
Magnemite learns Recycle from a Move Tutor in Black 2 and White 2, Omega Ruby and Alpha Sapphire, and Ultra Sun and Ultra Moon. Wild Shuckle can hold Berry Juice.
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How about this: Mewtwo vs Mew.
You never said he had potions
Gooooooooooooooooooooooooo