We've Killed Atheon in one time's vengeance without any gjallahorn. We just all sniped him in his precision shot and then shot him with rocket launchers in the last few second. He teleported us but we just nailed him a few more times and he died. Honestly it doesn't take that much effort if you have a good team. They didn't cheese Atheon, they just used an exotic, best of the best weapon, to take him down quick enough.