The damage from demolishers was negligible for us and just sent a DK tank, myself a MW, and 2 DPS as soon as the doors opened. This way we only had to do two towers and get into 2nd phase asap. As for the cone they just need to watch their feet better it sounds because it's easy to spot, or alternatively watch him and the way he's facing. Standing closer to him makes it easier to get out of as well. Then just left 1 DPS up there to kill off proto drakes the rest of the fight.