In fact, I consider it not much harder than any other bonus location, if you plan your actions carefully. For comparison, being assaulted by a group of ogres in Arcadia or hordes of MaxHP draining enemies in Turdak's Sanctrum is a much more terrifying sight.
Also, I've finally defeated him :D
Just for the sake of doing it, since it's easy to grab all stuff on the floor without putting up a serious fight with him.
This time I've fully assembled my strategy just in time, and focused on draining his mana with my ice attacks. He did spawn with weaker minions this time, and I ended up only moderately wounded.