The Cinderstorm fight is a bit of a DPS race, but more importantly is about movement.

Cinderstorm – Must line of sight the boss and every other player
Motes of Flame – Move into the room and avoid touching

Cinderstorm – Fire AoE pulse around the boss dealing significant damage, must be line of sighted
Mote of Flame – At about 25%, a circle of Motes of Flames with lasers between them will move in toward the boss

After killing Captain Black Spit, be sure to release the other adventurers from the cage so that Comda will solve the puzzle for you which spawns Cinderstorm.

Cinderstorm appears in a corner tower with a doorway and 3 “windows”.  Everyone but the tank should be assigned to one of these openings before the fight begins.  Planar protection and fire resistance on the tank can be useful.

The tank takes the boss where he stands and 3 fire lasers will appear.  These lasers originate with the boss and radiate out toward the walls, rotating around the boss.  Avoid these lasers as they deal damage and have a slowing effect.

Players can stand in the doorway to avoid the lasers, but should not stand out in the hallway or they will be instantly killed.

Periodically, the boss will cast his Cinderstorm and begin to pulse a red AoE effect.  Everyone but the tank must line of sight the boss, using the assigned 3 windows and the doorway, but unlike Expert Mode, each player must line of sight all the other players as well.  This means the player using the doorway must go to the right side of the doorway, not the left.  The tank needs to line of sight the player in the doorway.  The player using the middle window should use the left side (when facing the boss) to avoid the player in the doorway.  Players using the windows should also take care not to fall during this movement.

At about 25%, a ring of Motes of Flame, with fire lasers between them, will originate at the outer edges of the room and begin closing in on the boss.  Everyone must move in to avoid these lasers, which will instantly kill if touched.  This enrage mechanic gives a limited time to complete the last 25%, so DPS becomes paramount.