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This is in the trope description of The Computer Is a Cheating Bastard: The computer player is a cheating bastard whenever the "rules" differ between you and Video Game A.I.-controlled opponents. My Rules Are Not Your Rules is when the game pretends it's not a cheating bastard, but really is, but in your example it's just blatantly cheating.
SNK Boss is about bosses who do not respect the Competitive Balance, not sure if it fits.
Oo oo ah ah
The Computer Is a Cheating Bastard is fair, I admittedly should have thought of that. Thanks everyone.
A video game boss that is allowed to go past otherwise established limits of the game, such as having a larger-than-otherwise-allowed party size. For example, the ax party size for the player and all other units in the game is five, but this boss gets to have six.
Couple of spoilers ahead for Pokémon Legends: Arceus and Unicorn Overlord: (Seems that spoiler tags don't work here)
I don't think it's Beyond the Impossible as this is more a violation of a game mechanic than the "internal logic of the story". It toes around Boss Bonanza, but in each of these cases, it's not a series of bosses, but one boss with a larger-than-standard party size to use against you. Maybe there's something to an Inversion of Fair-Play Villain, but I'm hesitant.
Edited by BeerBaron