You absolutely have to dodge the water spouts.

Then during the transition you MUST CC the casters, or dps 1 and cc the other. While the tank holds threat on the kill caster + melee add.

I forget the exact name of the adds, but I made macros /target AddName /cast Hex on my shamans. I had 2 shamans more to the left during the transition so that they'd target the left, and the other 2 targeted the right.
You could also use /targetenemy considering the casters generally spawn off to the sides.