I’ve done all dungeons solo, and I’m not a very good boss-fighter. A competent thrall is very, very helpful in some dungeons, and even with one, I skip the Arena Champion fight in Warmaker’s Sanctuary (which, luckily, is completely optional anyway).
There’s one miniboss in the Sunken City which may be bugged, at least it seems to have a million times more hitpoints than anyone else there. But you can just run in, poke at the recipe tablet in the room, and run out, without killing that dude. Everything else down there I managed to defeat without a thrall (even though in the final boss room, some minions started respawning before I had finished the end boss because my good weapons started to break so I had to kite her with whatever pokey-stick I had left).
The Wine Cellar’s final boss is challenging, but again, a good thrall will help a lot. I haven’t tried to beat that guy without a thrall. I suspect it would be a long and tedious fight. Theoretically possible, yes, but not sure if my skills would be sufficient.
In Solo mode, of course, you can use sliders to modify the damage inflicted and taken by your character and NPCs, so the difficulty can be adjusted to suit your playstyle. In my Solo game, NPCs (including my thralls) do only 50 % damage because, like I said, I’m not a very skilled player - but someone who is better at timing their dodges would surely manage those boss fights even with normal difficulty levels.