10. Mitch Conner - South Park: The Fractured But Whole
We'll give South Park: The Fractured but Whole some measure of credit here - the game is entirely aware that one of its toughest bosses is a total cheater, and clearly has plenty of fun mocking that fact.
Mitch Conner, the demented alter-ego of Eric Cartman's left hand, flat-out decides to ignore the game's firmly established rules during the player's two major encounters with him.
Mitch will randomly apply status effects to make the player slowed, grossed out, or burned, while also being able to reflect the player's attacks right back at them. In the second fight at the end of the game, he'll even teleport around the area on a whim.
The other kids call "Mitch" out on his blatant cheating, of course, but he doesn't let up until you finally beat him.