He aggros as soon as any member of the part walks onto the central bridge at beginning of zone.
Simple tank & spank. The knockback effect he once had was removed.
As of 12/24/2012 I can attest that there is still a knockback. I tried to solo him as a monk at level 93 with a healer merc, but he knocked me off the bridge. Perhaps it appeared that the knockback was removed because it was moved to be so late in the fight that anyone with 2-3 people could kill him before it happened. I suggest you pull him back to the door so the knockback won't put you in the abyss.