Have you beaten Atheon without Times Vengeance? I know we used to push Atheon off before the first portal teleported, but just putting it out there. Or have you tried going back through the entrance doors during Gatekeeper? Like if you go into the past where it's open you can open it in the present for the rest of the team. I would try not to overcomplicate it. The method for 5th chest was simple. It was really the location that was tricky.