Quote Originally Posted by Dano View Post
no, he is correct, I can confirm dreadwash (unavoidable mass hysteria) is stunnable but dreadstorm (aoe hysteria) is not, when you try to stun dreadstorm you'll get a "Invulnerable" warning. Getting dreadwash stunned is much more important when the guard rails are down during the last 15% phase of the fight.

the easiest way to deal with that add is do it like dreadknights in T5, stunlock and burn it down. Only start stunning when it's below 80% health, too early and you'll end up getting resists, other melees should refrain from stunning until after the 3rd stun.
I agree with this as well, in fact I wait to do the second stun UNTIL the cast bar for dreadwash appears, and I have never had the mass hysteria effect by doing it this way and can confirm it is in fact stunnable. If you experience otherwise somebody else may be stunning and have built up the immunity, or the stun may have missed (accuracy below cap). Also, as Dano said, easiest to start stunning at about 80% and just keep stunlocked, but keep in mind dreadwash is the priority and dreadstorm is survivable with careful awareness.