Level 18: Factory
Get inside and call the elevator
After Valerie stops driving the forklift, lie down on the road and scope out the building ahead. There are half a dozen enemies clustered around the entrance; you should be able to peg one or two by shooting the fuel tank to the left, then pick off the rest with your sniper rifle or OICW as they cross the bridge. Dont get complacent, though; there are two more heavily-armored soldiers near the elevator itself. Luckily for you, your machine gun makes short work of pretty much anything, armor or no.
Open the next gate to help the forklift past this section
What follows next are a couple of firefights in large, multi-level rooms, but luckily, Val and the nuke will be hanging back, letting you proceed at your own pace against the multitude. The first room here contains eight or so mercs and scientists, the bulk of which will already be either on the ground level or coming down the stairs to the right of the entrance when you make some noise. Set yourself up along the left side of the entrance tunnel and start firing away; if you have to reload, draw back around the corner before doing so.
The next room down is accessible only via the two entrances on the top level of the catwalks. This creates a great choke point to set up an ambush from; just walk into the room, throw a grenade or fire off a few rounds to get everyones attention, then draw back and lie prone in front of the doorway. Itll be somewhat dim, so save your night visions charge by only hitting it when you hear footsteps approaching the doorway. When things calm, let your night vision charge up again and make a sweep of the room before unlocking the gate.
Open the gate to the Mutagen Chamber
The last hallway here contains a few more armored security mercs, including one with a riot shield. These guys wouldve been a serious problem had you not picked up a machinegun earlier, but now, theyre just another potential corpse, except that they drop 30 armor instead of 10, like their less protective counterparts.
After Valerie parks the nuke, run inside the mutagen chamber and shoot up with the mutagen vaccine, which, like all vaccines, comes in a case with a big biohazard symbol on it. Nothing suspicious about that.
Protect Val while shes arming the bomb
Itll take Valerie a minute or two to arm the nuke; during this time, shell be vulnerable and wont be able to fire at attackers, so itll be your job to fend them off by your lonesome.
Youll be facing off against at least four waves of mercs, many of them wielding riot shields. Theyll approach from two locations, one of which is the small door with the red light above it across the way from the mutagen chamber. The other waves will come from down the hall. These locations will alternate as the origin of waves, so if you fend off a charge from one location, reload and turn your attention to the other. The nearby door can be adequately camped with your machine gun, but the hallway is long enough for its inaccuracy to come into play with the soldiers approaching from that direction. You can even the odds by chucking smoke grenades into the hallway and using your heat vision to shoot through the fog.
Get the hell out of here. Val will follow you
Do like the man says and start running! Valerie can unlock the door adjacent to the emergency room from which the soldiers were coming, so head through, clear out any enemies and book it back to the surface.