You have to make sure that you have projected textures enabled so you can see the circle around the tornadoes. Else you will be picked up. One thing that you can try is to enable broadcasting: move forward, and then either strafe left or right, slowly making your way to the other side. Note that you don't need to make it to the other side, as long as you are in range to damage him you can stay on the same spot (provided your healers can take care of your dmg taken), but healing rains and healing totems could fix that. It's pretty much all about execution in the last phase, and even 5 man can take him out in that last phase.