Sorry if anyone has already said this/
But the enemies behind the wall before the bridge extension that you pull the lever to have "Crimson" in their name. They consist of a pack of the war hounds and a flamethrower guy. So they are most likely void as well (if we are going on the different elemental theory).
This would still leave solar out there though...
Maybe only kill the flamer and run around with the dogs alive and see if the other door opens after a while?
Dont remeber the layout of the mission exactly.
But maybe lure the dogs into the drill with 1 person and have the other activate the drill.
I know it doesnt activate if a player is inside,but it closes as soon as you exit it.
Maybe trapping the dogs inside and triggering something.