Here's the three battles that took me a good while to figure out...
1. Draconis - freakin misleading, invising, brown dragon menace
2. Gromnir - first boss fight, didn't really know what to expect. Once i eliminated both mages in the opening salvo, this battle was much easier.
3. Melissan - purely for the length of the battle, 7 battles in one.
and on the other side of the coin, here's the 3 battles that i thought would have presented far more of a challenge than they actually did.
1. Demogorgon - made paste of him in a minute flat. I expected him to be the hardest thing in the game.
2. Abigazal - his door guard makes him look easy.
3. Balthazar - what a letdown after some of the epic battles to get to him.
4. Ravager - i found a handy tactic on this one. I gated in a demon who attacked the swords, who couldnt damage the demon. Left me to have my entire group deal with the ravager.
One other thing, wouldnt you expect Melissan to be a heck of a lot harder than she was at the endgame? I mean, i fought some mages that had 6 or more protections to breach before i could get a hit off. She just had a blade barrier and a stoneskin. In 5 seconds flat i was greater whirlwinding her to toast. The only hard part of that battle was the 7 consecutive battles without being able to rest. That fallen solar was a pain too till i breached her defenses.
The party i had was:
Me (cleric/ranger)
Minsc (and boo)
Jaheira
Nalia
Aerie (or should i say, ritual sacrifice, since she died every battle if she got hit)
Imoen (who, for a 6th level thief, became quite the respectable mage by the endgame)
Guidar
19th/17th cleric/ranger