Brackish Vaults, Duo, Frigid Shatter AOE way over 15m

This is so funny, been doing that zone for the duo weekly every week for while now and I never knew that mob was supposed to root itself. As far as I can tell, it has always appeared to move with me across the platform while casting. I just though the fight was designed to be a straight up DPS check. You killed it before it cast the Ice Storm or your dead. Only thing that made it tolerable was the merc rez. The merc would not die to the Ice Storm, let you catch the rez and finish the fight.

The one thing I noticed was he didnt give off his casting warning and telling you to move. So it as rough to judge when it would go off.