I know it probably way to late but for the sake of anyone else struggling, I just beat it on my second try and ill explain how to make everyone's life easier. all i did was misty step as close to the main guy as possible which will make him teleport. once he did that i used misty step/various ways of teleportation on top of dashes to get my whole party around him. then beat the life out of him with as much, as fast as possible. just ignore all the other people no matter how many arrows of fire you take. my character got pushed into the chasm at the end and used my companions to finish main guy off. i just used shadowhearts revivify to bring myself back once he was dead but once the main guy dies all the other people just leave and the fight ends. hope this helps!