Is end boss of Deepstone bugged or am I missing something?

[Tony.8061]

The last boss of deepstone when he goes to the center and charges up an attack we have to kill the enemies that move towards him, however 99% of the time he gets 0/10 stacks yet he still does the shockwave and hits the group from 6-10k damage, downing 2-3 people almost every time, even worse with frailty. Occasionally after he's done charging the attack he won't do the shockwave. Am I missing something?