You can go on and solo the Underdark as well. Only things I have not managed is the massive burrowed monster and the gazer as the space is extremely limited while fighting them.

Solo kill list: One stupid evil (displayed alignment) Githyanki warrior, a dark grove, a Zhentarim hideout full of "experienced" rogues, half a Goblin camp including two hobgoblins, two minotaurs, a bunch of underworld monsters and their wannabe sorcerous overlord