Apologies if this method is posted elsewhere and everyone knows about it anyway but thought I'd share just in case.
I'd already completed the fight between Balthazar's reanimated guys and the justicars, chatted to him and he'd sent me on my way. As I was leaving the room, I realised how much he'd annoyed me, really wanted to despatch him and also had a Cunning Plan. So I reloaded my save before the fight.
I'm a druid with Shadowheart, Gale and Astarion in my group. I stealthed my team and unchained them, sending in Shadowheart to initiate the scene. Once the fight started, I pulled my druid and Gale back into the mushroom kitchen, moved Shadowheart away from the combat, and sent Astarion sneaking in around the very edge of the room - he wasn't in combat - to lockpick the door to Balthazar's 'Office'. Then he opened the door and stealthed back to the mushroom kitchen. I gradually brought Shadowheart back into the same room and shut the door with lever.
Then happily watched while the two sides went about murdering each other, including Balthazar and Flesh. It ended up with about 5 justicars left, who opened the door to the mushroom kitchen, where I finished them off.
Hope this helps/amuses someone.
I flipping LOVE that the game let me do this.
P.S. Please let me give the Stuffed Owlbear to Arabella.
