There is a lot of ways to do it, but optimally I think SCH does the blue/red fire and WHM/Caster DPS (cos mDEF) or BRD move to WHM to pass the brand is the most optimal.

Asking any of the DPS to do the blue/red fire is an actual DPS loss. SCH's DPS is never going to match your BRD, and since SCH is primarily DoT based it is still okay. The loss is not as significant as the loss on any of your DPS doing it.