Yeah, this has always been the hardest encounter in the dungeon. Those four thralls do a crap ton of damage and to have all four of them on you while fighting the boss at the same time is really difficult. Now with that damage applied to the boss he goes down almost instantly.
The encounter is actually kind of annoying because they all respawn and if you’re trying to smelt some obsidian they will respawn several times before you’re finished. It makes it so that using the forge to get obsidian isn’t worth it. It’s far better to scrap obsidian weapons than to use the forge to obtain obsidian bars.
I thought this was an issue with my mod and fixed it there, so it’s a little cathartic to see that it was a base game issue all along. What’s happened is that at some point the community ID for either the boss or the thralls has been changed when it used to be the same. If you’re using mods you can fix this yourself by blocking the spawns with land claim and making new spawns with a community override so that they’re all the same. I know Pippi can do that, not sure about Tot.