The method of pulling Heigan so that the tank is on the front corner with all the melees and the boss is standing in the fire and casters on the opposite corner of the platform makes phase 1 damage output very nice.
Issues with this setup:
your tank has to position Heigan just right to avoid casting reduction on casters
Melees are forced to attack from the front of the boss
Pets are all but guaranteed to die from the fire (not that they can't be healed, but LoS gets them)