The Balthazar Tips
This fight was indeed a pain in the ass, and I had to try multiple times before figuring something out.
IF you have the iron flask on you still, I suggest tossing that over to the cliff next to the stairs. Balthazar seems to love running over in that direction during the fight, so the Spectator will be waiting for him, and will probably clean up some of his undead in the process.
Aside from that, Shadowheart’s Turn Undead should be powerful enough at this stage in the game to send the more powerful minions running, and the weaker ones will just be immediately destroyed. Just don’t use it around Balthazar, as he gives off an aura that buffs nearby undead with turn resistance.
I highly recommend bringing Gale with you, particularly if he has AoE damage spells like Fireball. I tried to do this fight with Astarion, Shadowheart, Lae’zel and Minthara, and regretted it because there were so many enemies ganging up on me at once, and I didn’t have an effective way to clear them out.