He will do a raid-wide stun at a certain % of health, then run across the bridge to the lab where you will fight a ring event of 3 mobs, and then Fitz himself. Does sometimes bug to which if there are alot of DOT's on him, he wont run across the bridge and you will have to fight him after the long stun subsides.
Watch out behind the raid as little book minions will continually come across the bridge from the library to attack after he changes sides.