Same problem here. Unchecking Show Damage/Show Crit Damage doesn't fix my problem.
I died on phase 3 of my first attempt. On my second attempt, I was about halfway through phase 3, when the boss basically fell off the platform into the middle of the ring. He was kind of hovering there but I guess he couldn't get back on because my character was blocking him. After awhile he stopped attacking and disappeared off the screen. Nothing else was happening so I finally quit the game and restarted. Once I came back into the room, the fight didn't restart. The boss is sitting frozen inside a sphere on the platform with no clothes on and his arms sticking straight out to the sides.
I have no way to progress my game or finish the campaign. I was an original Kickstarter backer. I love the game and have been playing it for years, but this is tough to forgive. I'm sure there was immense pressure to release the game, but clearly there needs to be a way to reset any mission so there's a way to get around problems like these when they occur.
I'm reinstalling the game now in the hopes that will fix it. Either way, devs, please hotfix this asap. Game stopping bugs shouldn't make it to release.