Fire eles are hit and miss usually for me. I get to max range (close to where the stairs are going toward him) then drop all my totems. I lay down 4 fire eles and my totem of wrath. The fire eles don't go after him until he runs toward your team. I have had some times that all the fire eles do is stand there and others where they will take him from ~30% to 1% while I am swimming around. I have also summoned a fire ele to close to him and made him despawn.

Just make sure you have some sort of star chain heal setup ( ie every toon targeting another toon and then casting chain heal). It is actually simple once you figure out how close you can come to the orbs.