Have managed a couple of times to get to phase 3 on round 9. Voriak skulls easy. CG easy. But the real bosses, imo, are the clannfear often standing in the way when "circling" the CG and the wraith that often seem to float by when taking down the CG.
The Clannfear is often killed as collateral damage when destroying the CG. And the portals often stop working, no longer glowing, so must often run to 1-3 portals before being teleported upstairs.
Another killer is fall damage when jumping down. Finally, not that easy to prepare for the return of Voriak on the ground when not knowing when my DoTs destroy the last crystal. Have been killed because of that, because didn't see that Voriak had moved downstairs again.
This really is a game of situational awareness
A question: when using a portal will I always "land" in the same place on the circle upstairs? If so, where exactly do I "land", above which sigil approximately? And which crystal should I destroy last in order to jump down close to the shield sigil? In other words, should I run left or right after being teleported upstairs?