OK maybe you have to keep the back door from closing. When you start the gatekeeper leave somebody on the stairs throughout that part. When Atheon starts then all 6 gather on the steps where you kill the gatekeeper. 3 get teleported so 2 can open the gate and 1 can stay on the stairs keeping the back door from closing. Rinse and repeat until atheon is dead.
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Door closes regardless of someone being outside of it.
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The door closes even if someone is out there. Plus, the person out there CAN be teleported.