Belloq in Twin Giants - Azteca and Tymen Whiteflame in Basion - Khrysalis.
Belloq because his usual cheat plus the addition of triage to his minions' abilities is just a pain making the Fire Sprite solution from the previous encounter useless.
Tymen Whiteflame because she casts Mana Burn every 3 rounds. This makes it very difficult to solo, unless you have a really high Ice resist and a load of TC Balance Dispel.
Cipactli in Azteca was the bane of my existence for some time, almost had me to the point of quitting. Jabberwock was also hard for me. I haven't fought the Tentacles in Aquila yet, but I'm guessing if and when I do they'll make the list.
