So I ran it off of the Martian Incident (in fact, it takes place between DOOM 1 and 2), insted on a UAC Mining and Research Facility in the Asteroid Belt. I assumed the Marines weren't employed by he UAC (there was CSEC [Corporate Security] for that) but there to monitor and respond to the facility. The Marines' Company went in in three waves, the players' platoon in last and they were left to guard the hangar as a Company of Space marines [approximately 250 men] go in.
Over the radio they hear contact and combat with UAC CSEC, which while rare has happened before. After a number of hours they hear the radio go silent and the sergeant goes in with the corporal, and a Private Garcia. Leaving Captain Dutch and Private Johnson to guard the dropships.
They go in and find bodies, move in and in the next few rooms find around 120 of the Company line up on the wall, some in body bags, others stacked up by the wall. But silence. And a lot of CSEC bodies as well. Spirit rolls. None of them have seen heavy combat. Or mass casualties.
In the third room they find their Platoon leader propped up in a corner with a pistol and a radio, but dead with a shotgun wound to the chest. The sergeant checks him and finding the radio was dead tosses it away. The Corporal (Chaingunner) sees the "Lt." stir and lift his pistol, he then fires and hits Garcia in the face (I rolled a 19), and three other shambling undead stand up. Still stiff and uneven their shots miss mostly in the gunfight but the pair of Marines overcome the dead. Spirit roll(s) throughout the encounter.
They then check the side rooms, finding a CSEC Sergeant with shotgun waiting for them but put him down easily. They enter the tram station and clear it, with a dynamic firefight, and head to the next station. Spirit roll since now they have time to reflect on their dead friends and zombies. The Sergeant orders the dropship to go and get reinforcements with their dropship. They are moving from "Reception" to "Maintenance and Housing." on Level 1. They move into one of the housing halls and begin to clear rooms, they split to clear opposite rooms on the hallway. The Sergeant smelling brimstone and encountering "flaming teleportation" and an imp who takes a moment to shotgun to death. The Corporal finds a buff zombie and one wielding an assault rifle. The fight attracts a Spectre and the other enemies in he hallway as they search, leading to the Sergeant catching fire and the Corporal getting badly clawed. They find some medical supplies and ammunition (which thus far have been scarce). Moving into Maintenance Storage, the Sergent nearly gets killed in the Computer Room by a sergeant, only surviving because it had to reload its shotgun (I rolled on this) and left the Sergeant without effective armor. They heal up and the session ends.
Easy thingsalmost kill them, hard things (demons and imps) die like chumps mostly.