The only one I had a little trouble with was the Undead Dragon, which I think is supposed to be the Last Boss of the game, given the order in which the staff suggests you obtain the items needed to escape the Exiled Lands.
However the Black Keep / Barrow King still give the best rewards for defeating them, and they are quite easy.
The Undead Dragon though has a fair amount of HP and does hit very hard, especially his breath attack, but if you can get under his belly you can sort of glitch fight him. I feel he is the only boss designed to not be fought alone, but you can still kill him alone fighting smart.
Honestly my biggest issue was avoiding the respawning skeletons as I chipped away at the Undead Dragon’s Health, they caused enough of a distraction to allow it to get a few very good hits in on me.
Perhaps this fight is harder than it needed to be considering I didn’t use end-game armour to kill it, just the Conan Royal Armour, perhaps had I gone in some heavier stuff I could have taken it easier, like tanking Death Bringer in full Silent Legion.
Of course the other nasty effect of this guy is, that you need to run through quite a bit of corruption to get to him, so you’ll never fight him at full HP. I think all boss rooms and dungeons should slowly give you corruption to stop people playing the run away and heal game, give them pressure to actual win the fight in a reasonable time.