Any endgame bossfight where you suddenly have a whole new form/vehicle/power that you haven't had the entire game and that you will only have during this fight that completely changes everything about how you play. Like the fight at the end of DMC where you go full demon for the first time and fly around or like in Extermination, where you have to use the turret guns that you haven't seen before then. It's like the game is saying, "hey, you know all those cool moves and equipment we've had you master this whole game? F*ck'em, now you shoot lightning from your hands and you're a wombat"