have firebrand healer kite / heal it's easy to do. That will put one more dps full time on firebrand. Also having the tank on stormcaller move during double destruction instead of the dps helps alot also. You can also put the marauder in the group with all the dps for the damage buff. If that doesn't help then someone is seriously derping it and you need to gkick them
. I'd suggest running mox dps parser. Find out who your problem is if it persists after the changes i said.