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Once per turn, during the start of the Damage Step that this card battles an opponent's monster with higher ATK than this card: You can have this card gain 400 ATK

Check out this vintage Pokemon TCG commerical below.

This cheap green combat trick is widely used in Commander, and that includes Elf tribal decks such as Ezuri and Rhys builds

Those times when Counterspell wins the game but Mana Leak loses it stick with you and make you resent the Leak

Don't get me wrong, though

Since nearly all games (including all official tournament formats) are between two players, this is an instant win for the player who activates it