Discuss Divinity Original Sin, as well as comments posted to our Divinity Original Sin Wiki and Divinity Original Sin 2 Wiki
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If you can't get the painting from dialogue, just steal it and kill the trolls.
Cryogenic Stasis saved Arhu in my playthrough. For some reason he was permafrosted the whole fight (not just for one turn), which turned out in our favor cause enemies just couldn't leave him alone and always focused him first, killing poor fellow in one mass source spell (Meteor Shower from Kemm or Hail Storm from Black Ring mage)
finished the fight in 1 round, Kemm didn't spawn (or it was because I already killed him upon entering Arx...)