This is supposedly a two-stage fight where Braccus Rex will bring in the Kraken when he's injured, but the second stage can be skipped entirely if you nuke him down in one-shot.
The setup is pretty straightforward:
1. Have one character (Lohse in my case) that has 70 initiative (we need this so it'll act first before Lucian who has 69 initiative); Peace of Mind + Large Wit potion are great here.
2. The first character should teleport everyone into a cluster and then setup Blessed Smoke Cloud at the end which will turn everyone invisible.
3. Delay all your characters' turns; enemies will skip their turn because they can't see your party.
4. Delete the enemies in one-shot. There are various builds to do this - I use an explosive trap build (covered here: ua-cam.com/video/MRv1Qb-WG1U/v-deo.html) which throws 9-10 traps and blow them all up with a fireball. Lucian might survive the explosion, but he won't have much HP left so finish him off however you want.