Walkthrough: Drop down to the train tracks and start moving forward. It would be a good idea to examine a human body along the way #1, because you'll find a lot of ammunition for all sorts of weapons. Turn left after you've reached a junction #2.
You're going to be attacked by two monsters just around the corner #1. Be careful, because they're very fast and they can use tunnels to perform surprise attacks. Aside from the beasts you'll also have to watch out for large holes in the ground #2. Dropping inside a hole will result in the death of the main character.
Make sure that you've eliminated both creatures and then look around to find a new body, as well as a lot of ammunition #1. Head to the area where you first saw one of the monsters. Examining a new body will allow you to collect ammunition and a pneumatic rifle #2. Remain here and examine the third body found on the other side of the grating. Doing this will allow you to collect 2 rounds of gold ammunition.
Return to the main tunnel (the one with the train tracks) and start heading towards a metro car seen in the distance #1. Enter the carriage and look around to find a new body. Pick up ammunition #2 before leaving the carriage.
Continue exploring the area until you get to a new junction. Check the body, as well as backpack found nearby #1 to collect a lot of standard ammunition. Choose a corridor found to your right #2, BUT watch out for traps located in the area. Prevent the main character from touching the hanging cans by moving slowly and close to the left wall.
You may enter a new room. Check one of the nearby cupboards to find 3 rounds of gold ammunition #1. Other cupboards contain standard ammunition #2 and you shouldn't ignore them, because you'll need supplies for the next battle. Exit the room and wait for Bourbon to disarm one of the traps for you.
Proceed to the next section. Approach the door where your colleague should be waiting for you #1. Stand to the left of the door, turn off your flashlight and wait for one of the guards to enter this room #2. If you want to kill him yourself you should use a weapon with a silencer attached to it or a knife to prevent the alarm from being raised. If you don't want to kill him you can wait for Bourbon to deal with him for you.
Enter the next room carefully and make sure that you're still not using a flashlight. The area is being patrolled by several soldiers and you should decide to eliminate at least a few of them #1 #2 using a pneumatic rifle or a silenced revolver.
Sooner or later your opponents will find out what is happening and this means you'll have to engage them in direct combat #1 #2. In this case you may want to consider using a normal submachine gun or a shotgun (assuming you can get closer to your enemies). You should also start throwing dynamite sticks to dispose of larger groups of enemy units.
It would be a good idea to examine all bodies after the battle to collect much needed supplies. There's a small fireplace in the center of the room #1 and aside from standard ammunition you would also find 20 rounds of gold ammunition. Approach the next door #2 when you're ready to move on with the mission.
Take cover behind one of the larger objects as soon as you've entered a new corridor #1, because enemy troops will show up in the area. I would strongly recommend using dynamite here, especially since a large group of enemies will be taking cover behind sandbags #2.
Proceed to the next area, however watch out for additional enemy units to your right #1. Make sure that all bandits have been eliminated and take your time to collects supplies (especially ammunition). Interact with a nearby door #2 to end this mission.