For a couple of weeks I've been running VoG on HM with my clan for the Exotic drop chance and I still need a Vex anyway. Whenever we get to the Atheon part we always give the reminder, no grenades, no rockets, turn off explosive rounds, etc... and sure enough while a number of us are trying to use black hammer Atheon starts moving around. At that point someone just has to ask why he is moving and it is always the same guy saying that he is using Gjallahorn, plus he usually screws up jumping out of the bubble and blows himself up. So we finally convinced him to stop with Ghorn one time which gave us somewhere between 3-5 black hammers with weapons of light and we took him down in a single teleportation cycle. Afterwards the guy commented that he finally got why we didn't want him using Ghorn. His whatever damage he was doing for himself vs 3-5 people with 20,000 damage a shot.