I repeated that quest now a couple of times. and in hangs always at a different stages. The only possibility to do it was to take board and non-weapons ot avoid riposte and click off DS. The slightest damage at level 115 bugs out that script. This was an annoyance at level 110, now it is really nerving. The strange thing is that the reason "riposte/DS" is no real explanation because the task steps updated, only the named didn't spawn after the successful kill steps of the trash mobs. This happend at the first stages where the Amigdalan should spawn. But this happened also at the end steps after killing the first named, no other attacker spawned anymore.
I ran into this issue also. The solution is leave sufficient time between the 2nd to last and last update within each step. Whatever step(s) are required to trigger the next spawn. If the temple or ramp phases are cleared too quickly then the mission can't be completed.
