Temporary work around for those experiencing the bug:
Clear the entire area from the room after the Mentor Assassin group all the way to this boss. Have the tank pull then get the whole group to run all the way back to the room after the Mentor Assassins (do not go past the door to where the Mentor Assassins were or the boss will reset). During this time, the boss shouldn't have started spawning any adds yet. Once the boss reaches the room, engage him. He'll go immune for 20 secs or so while starting to spawn the adds. Wait for it to wear off then blow all cooldowns and burn him down hard. You're aiming to get him to 25-30% by the third spawning and he should die just a little before the first adds arrive at your position. Quickly run back out the door and wait for all the adds to evade bug and reset, then go back in to loot and continue on your merry way. :)
This boss will randomly spawn a copy of your party, that is ccable. The boss himself is immune to damage until you kill the add.
Unfortunately, as of 1/22/2012, there is a slight bug where he will spawn several copies (He spawned about 8 elite copies of myself tonight, which i doubt is intended)