Two large rolling fireballs appear when this boss is aggroed; they will circle the upper and lower sections of the room. Standing in the center section (the same section Ilsundur is located) will allow you to avoid these balls.
He sometimes sends you a Firebomb, which will explode 5 seconds after you receive it. You must drop away from your allies so that you and your team mates aren't hurt. (Henchmen don't know how to drop this bomb, so avoid them.) Heroes, however, can be ordered to drop it as if it were a RitualistItem Spell, by clicking the "drop" button where the target lock button normally is.
A Migraine Mesmer with interrupts and degeneration skills such as Conjure Nightmare is a good choice. In combination with a Broadhead Arrow ranger with interrupts, the boss's casting will be almost totally shut down.