What Base Game Boss is Most Unfair

Which boss in main game is the most unfair to you. Doesn't have to be a "hard" fight, just one that has some ridiculous stuff that makes it an awful experience.

For me is has to be Renalla. She's likely the easiest main game boss considering she can be staggered by a toy lightsaber and has less HP than your average late game enemy, but her summons can be absolutely terrible especially if she chooses to chain a dragon into a giant into another dragon.