If your group have ilvl 110 weapons you can easily Mage LB3 the first dragon and the boss and get only a single dragon (by getting the boss under 47% before the 2nd dragon). Alternatively, if for some reason you don't have LB3 (it has happened to us several times), you can always burst both dragons and use Melee LB3 on the boss during the second dragon.

If that doesn't work, leave the group.

Also do as they suggest on previous posts and evaluate the DPS by checking how many meteors you get during phase 1. You should get 4 unless something goes really wrong (then you might get 5 every now and again).