A buddy and I just completed hero's challenge last night. Maybe it was because we used henchmen, but it was impossible to pull groups of less than eight, not including the azure shadows that randomly pop up.
I think we got a bit lucky because we pulled a group of eight off, died when four to five shadows popped up, rezzed in between the first group and another group of eight, and squeaked by between them and killed the boss to complete the quest.
R

