Here is a video of the first 43 minutes of what you should do:
What I didn't do here and ended up coming back and doing was Addle, Wither him down while the tank continued to attack him. For some reason my ps4 stopped capturing.
If you can dispel his regen, you can leave and level up other characters, save, rebuy mats if needed. I ended up doing that twice. Once because I needed to lvl Fran for Wither and Addle and the other because I was really tired the next night playing.
For reference you can see the last 90 seconds of the fight, he doesn't hurt as much since he's been withered down a lot:
Here is the ending cutscene when you go to turn in the completed Hunt (spoilers):