It took 22 deaths but I figured it out....as a ranger...I had to constantly kite the Boss mob in a big circle.....joust/run away when the day/night (missiles) shout went off....and constantly run in a circle around him after the "massive laser" shout.......you have to continually kite to keep your merc alive also....if the merc is anywhere near him when the missiles shout goes off..it will die...
When the balls are up...get on the far side of the center from him...and continue running along edge of the combat area....
Hope all this helps...good hunting! And Nevah give up.......