I an playing as a solo sorcerer(DD), din't had much problem on base game, but on the throne of bhaal, with bosses behind an very long conversaion(that you can't skip and can't save after the conversation, making retries very costly) and very long bosses fights that can use infinite spells, summon multiple creatures without any limitation and be immune to time stop(how this is possible?), was very hard to play the thone of bhaal, this to not mentions the ultra small "arenas" that you can't even leave by no reasons. Took hours to beat Ravager, but Melissan, even following guides, i can't do anything. Slayer form? No damage. Black blade of disaster? He dispels you and ... No damage. Lower his magic resist with spell trigger + lower resist? Spells still deal almost no damage while she can summon an army of Death tyrant to kill you.
Aparenlty there are an "puzzle" involving the scenario to deal with her and sorry but i usually hate puzzles. Maybe i will put on story mode, only to see the ending because this is not fair. Bosses should magic should work in the same way that players magic works.http://www.sorcerers.net/Games/BG2/Walkthrough2/ToB/chapter-10/chapter-10-overview.php
Mephistopheles on hardest difficulty on NWN1 - HotU expansion was an piece of cake compared to her. The Dracolich too. And the best part. On NWN1 i can save in fights, so i don't need to see the same dialog 500 times if i need to re try 500 times. And even this epic bosses, operates under the same rules of the players, without gimmicky scenario BS.
Let her in near death(because she doesn't operate under the same logic of you MC) without killing her to try use the scenario to disable an power that you supposed to have too is extremely silly and makes no sense. Throne of bhaal is nothing compared to the base game IMO.