At the end of the mission, do not deactivate the barrier in the room that Johnson and his Marines are being held in. Instead, bring your partner to the part of the building near where the Iron skull can be found. Get your companion to keep walking up the rock so they can jump onto the highest part of the building's roof. Once they're safely up there, kill yourself so you can respawn with them. If you start slipping off, quickly walk towards the rock and jump onto it, walk up it, and jump back onto the roof and you should be fine.