My trick for Balthazar is to fight him in his original room. Don’t fight him in the nightsong’s room.
If you fight him in his original room you just have to deal with 3 ghouls that are a joke and his big bruiser plus Balthazar himself. Balth is easy to down in a round or two depending on your party comp and then you can just clean up the remaining undead. Once I discovered this strat I’ve never fought him in the nightsong’s room since.