You have to stop the templar from teleporting. My fireteam did it last night after a few tries. Its a fun, and different way to do the encounter, as well as getting you that 5th chest. Basically you attempt to keep his shield down at all times by having people stand in the circles that tell you where he is about to teleport. This essentially stops the Templar from regaining his shields, however it does make him call for his Minotaurs, along with the harpies that are already flying around. It was fun, and added a small amount of difficulty to an otherwise easy fight.