I actually had a lot of trouble with this boss. I think it is bugged. It may have been related to my build – I was using a necromancer with a lot of minions.
I did the thing where I had to stand in the white circle and use the special skill. The boss would lose about 75% of its health, but then it would regen back up to full health before I could use the skill again. I did this for 15 minutes (!!!) before I decided to try killing off as many of my minions as I could. After I did that, the boss stopped regenerating to full, and I was able to kill it after 2 or 3 uses of the skill.
My guildmate said the boss should’ve died way quicker than that, so it looks to me like there is some problem with the minions.