https://wolcengame.com/forum/bugs-issues/bug-reports/push-fix-for-the-act-3-invulnerable-boss-bug/

Here is your answer. Good luck bud!

I read all the ways to work around it and none had worked for me... so I tried my own method. I was on normal settings which states "How Wolcen should be played if you want the Wolcen experience" or something of that sort. You can change it in the main menu. I changed mine to "Story Mode" to give it a try... what do you know! I was able to go through all 3 phases, no problem. Note: This happened after numerous tries on Normal. Maybe I got lucky, maybe it worked. Give it a try and let me know if you are able to complete the game. I was a happy camper once I was finally able to beat it.