If you're still short on DPS by a bit, have your SMN cast the LB3, instead of your monk.
They're the best candidates for DPS LBs because they lose the least DPS during the long cast animation. As long as your DOTs are up, and fester is used right before LB3 is casted, you only lose damage from Ruins.
Also, don't waste too much damage on the Add, it will most likely die from a single meteor at 60% hp. Everyone should switch back to the boss. If the Add is still alive after meteor just finish it off normally.