You cannot use main doors, so head to a huge rep pipe. It's hard to miss. Get inside and use a set of doors to get in. Head up the ladder and through the next door to find yourself in a vast hall. You cannot open a door, so head to the control station – it's a room with big glass windows. Use the lever to let the vehicle in.
Note: In this level the armored vehicle will shoot aliens being really helpful. However, in narrow corridors you're still on your own.
Doors across the control station are now open. You can open only a door on the right, so head there. You're now in the next control station. As you step in, you will be trapped inside. You need to hack the panel to open the doors, but don't do that yet. Something will be attacking the door. Obviously, that will be an alien. Wait for it to burst in and kill it. After that you can hack the panel. Don't forget to pull the lever after that.
Now go through the next door. There are a few bugs inside. You might jump up, but they are harmless. The vent on the left is a dead end. When you go to vehicle doors control, two aliens will attack you from opposite sides. After you pull the lever, three more foes will attack you. Something will try to destroy the grille next to the door, but don't mind that.
In the next corridor you will see a dead body. After a few more steps you will come to a trap door which is sealed shut. You will have to find a blowtorch, but for now let's open doors for the vehicle.
Head down to where the armored truck is. There's not much of interest, but a piece of paper next to the control room. Once you open the door, you will see a small explosion and a few aliens will attack you. Get rid of them.
In the control room you can pick up a smart gun. It's a good weapon. There is no need to save its ammo for future right now, don't hesitate using it. And don't forget to pick up extra ammo for that gun next to a first aid kit.
As soon as you open the next doors, aliens will spawn. Armored vehicle will shoot some of them, but some aliens are situated in its blind spot, so you will have to shoot them without aid.
Move ahead and up the stairs. You need to go through the trapdoor, but you cannot for now. Open the door ahead instead. There you can find a dead body with a PDA and a torch in a box. There’s nothing interesting left, so move back and use the torch to cut two bolts. The torch is used just like your hacking device. Most likely, you will have to crouch for bolt cutting.
Jump down. You will be attacked this instant, so switch to some better weapon before falling down. Go down the corridor. There is some machinery on the left. You can scout it to get some pulse rifle ammo and flares, but you will have to fight two aliens.
Go up the ladder. There you will find more ammo in a box. You cannot go up, so move through the door at the end of the corridor. There is a chasm behind it but don't worry, you won't have to fight anyone there.
The next part is a small T-junction. Aliens will attack you, so get ready. Don't forget to pick up armor vest, then jump down to the vent. Inside there is but a single alien and a really straightforward path. Go ahead and up the ladder to find yourself in a huge section 3 hall. All doors are jammed, so find another ladder up and to another vent. As you will be moving through a round pipe (octagonal, to be more precise), an alien will spawn behind.
Drop down to get into the control room and kill an alien. Ammo box contains some grenades and more smartgun bullets. If you haven't switched to smartgun yet, it's time to do it now. There is also some health and ammo which will most likely be needed right now.
When you're done with trophies, pull the lever and prepare for alien invasion. Most of enemies will be killed by your friends, though.
Go through the doorway to the right of the armored truck. Eventually you will come to a ladder up and battle music will start. This part is tricky but not very hard. There are a few aliens up above you. When you come too close to the ladder, some of them will jump down. You cannot attack them from afar and they won't jump down being stuck above you.
When you deal with aliens, you can pick up a first aid kit right under the ladder. Flares are in the vent above, but you will have to use your all skill to get them. Most likely, it won't be worth it.
Anyway, the road head will lead to a scene where some guy is killed by an alien from the vent. You cannot save him. As you come to a small staircase, another alien will run at you from the corridor on the left. Get down and don't forget to pick up smartgun ammo from the box under the stairs.
Go to the main corridor. Huge doors will open by themselves letting the vehicle inside. You will see a smashed door to the left of the truck. Get in to see another guy getting killed by an unseen alien. You can't use the lever, so pick up armor and bullets next to a trapdoor and head down.
Eventually you will enter sewers. The voice will tell you where to go and where to turn. Since there is only one road and zero chance to get lost, you will easily find the way on your own. When you open the door and make a few steps inside, a bunch of aliens will charge at you. Climb the ladder.
Two more aliens will attack you. You can easily spot an exosuit. Sadly, there is no way to use it right now. After you search the area, you will find that all doors are jammed. Look for barrels next to a control room and shoot them to make a way inside. The security control panel you're looking for is at the end of a long way to the right. Use your hacking device and head outside to finish the level.