The boss is rooted. He dispels and silences players in his room.
The boss has 6 robed NPC's, 3 on each side of him in front of book cases, which spawn when he is pulled. These mobs spam heal the boss throughout the encounter unless they are mez'd. I recommend an enchanter.
Strategy: Have an enchanter outside the room, opening the door and mezzing the robed mobs from outside the room. You must have all 6 mez'd or the boss is healed. Have the enchanter focus on doing this and staying away from the door (outside, around the wall) to avoid silences.
You may leave other characters outside and around the wall to avoid the mechanics.
Tank and spank once the adds are being mez'd