technically not a boss, but...
in the sewers of the temple district, in BG2LSoA, tucked up in one of the corners, is a group of about five people. if even one of them sees you, then they demand 1,000 gold from you. if you refuse (or are unable) to pay, then they attack you.
I just faced off against them about an hour ago. Minsc and Keldorn dropped first, followed by Jaheira, Yoshimo, Aerie, and my main character. And the only reason Aerie lasted that long was a total magical AC of -3, and a total damage reduction of 16. It also helped that I'd summoned three skeletons and a squad of armed Gnolls in to help with the battle. I lost all those gnlls, all those skeletons, all the party, and only succeeded in killing one of them and "badly Wounding" another.
Too. tough.
beats me why those guys are hanging out in a sewer looking to collect 1,000 GP from any adventurers who happen to wander through (who, let's face it, are quite likely to be in short supply). These guys could be making a killing as dragon slayers.