Queen Alissa (Winter City)
Gytha, Jael'yn, and Elthara are all fights that are all pretty good for that stage in the game, but I dread fighting Alissa everytime and cannot fathom how she is even in the same dungeon. If you do not beat her in Initiative she basically removes a party member from the fight, and she heals 30% of her HP in a single round while doing it. There is no setting up against her, no tanking against her, and you can't even tease her to death.
You almost have to one shot her to not immediately lose the fight, meaning you have to dump all your stuns and ultimates on her until she dies, and at that point it might as well be a dialogue. By the Horns is a necessity for this fight since it does a ton of damage and makes it so she only heals instead of heals and beats you to death. Even then, if you do not split, act, or kill her before her 3 turns it means nothing. If you do anything but hit multiple extremely high damaging powers against her in a row, you lose.
Maybe I'm just getting worse with every new game, but there is no fight before or after her that is this much of a headache.