When TOB was about to be released the developers decided that the BhaalSpawn and Demogorgon were to hard so they toned them down to make them easier.
David Gaider from Bioware has released some mods that return these enemies to there Full and extremely hard difficulty.
I used to think like you did I had an unbelievabley hard Cavilier and no equal, but not now
If you want a challenge then download these Mods from
http://www.planetbaldursgate.com/bg2...es/index.shtml
and try to finish it by going solo with your character on insane difficulty.
(There is now a multiple Dragon fight. And Illasera is actually a tough opponent).
Happy gaming and good luck