This is my favorite guide I've seen so far to taking down Atheon. Note that the relic's cleansing aura also acts as a shield that can be shot through. A good strategy is to have the whole team meet up on the central platform after the relic has been retrieved. The relic holder can make a shield with the relic, while the rest of the team focuses on wailing on Atheon. Right before the shield wears off, a defender can pop a bubble and everyone returns to their positions. Thanks to Elytist for this strategy, as it is the one we used to clear Atheon. It was considerably easier than the exact method described in this video. Still, people who haven't fought Atheon yet should watch the video to get a feel for the rhythm of the fight.