I am going it alone. I am at level 11 character, play Level 12 (or 13 story). The boss is sitting/standing in front of a big gold throne when I enter the room, and the door closes behind me.
I know a cheap way of beating it. Just stand outside of the doorway when the door opens and shoot at the guy. He will eventually come towards you and you just stay outside of doorway and keep shooting him.