It seems, there's no satisfying way for a decent character.
I tried to stop him, but all the Droughs side with him instantly and they make short work of my party. Especially the sergeant deals a ridiculous amount of damage.
So I sided with him, which I didn't want to do. He got pushed into the lava, so I didn't even get his loot and still, I had to fight the whole dwelling. My party barely survived.
I tried it with my warlock, Shadowheart, Karlach and Tristarion. All level 5.
I made a deal with
the duergar elder and his rebels, killed the orb by luring it down a set of stairs and thus ensured the rebels' help.
That made the fight quite doable for me (level 5 party: Fiend warlock, Lae'zel, Shadowheart and Gale)
