Can you check Kongar out?
2 WB runs ago, I kept watching my hp go down until I got killed several times. I was standing almost a full screen away from him, so it could not be a ground AoE attack, as others between me and the monkey did not get affected. I thought maybe it was possible that I was lagging and did not ever see the red circle for the rock dropping, nor the rock itself, as there were probably 30+ players running it at the time.
Running it again last night, this same kind of "invisible" attack was hitting one of my teammates this time. There were only 10 of us, so lag was not really an issue. Neither I, nor the player hit, could see what was causing the damage. This player was also standing far enough away that a ground AoE would not be the issue, even standing close enough to me at times, that were it a rock, I should have felt it as well.