Walkthrough: Remember that you still don't have to be in a lot of hurry. Start this new mission by exploring the area to your right #1. One of the red crates will contain 30 rounds of gold ammunition #2, as well as standard ammunition and gas mask filters.
Rejoin with your allies, approach a new gate and wait until this new passageway has been unlocked #1. Make sure to put on your gas mask before heading inside. Check the area to your right again #2 - this time you can collect only two rounds of gold ammunition.
Continue exploring the area by using the stairs to get down from the balconies #1. You'll take part in a new monster battle here #2. Don't forget that you must assist some of your teammates so that they won't die.
Don't be too ecstatic after you've dealt with the first group of monsters, because more of them will show up soon #1. This battle will be even longer than the last one, however as long as you protect your colleagues you should be fine. Head on towards a new gate #2 and listen to what your team has to say about the current situation.
Approach the gate and hold the crouch key to crawl under it #1. Notice that the gate has closed behind you, separating your from your team (thankfully not for good). There's also a second problem - you'll have to take part in a battle with a red-eyed monster here #2.
This duel will be very challenging, because the monster will be able to kill you with only a few hits and it will also avoid getting shot by escaping to the ventilation shafts #1. The most important thing you need to know about is that you have to keep moving. As soon as you've seen the monster leaving the vents attack it using a shotgun #2 or a revolver. Aim for the creature's head and keep firing until it escapes again. Don't forget to heal the main character if he's about to die. Keep repeating all the actions mentioned above until the mutant is dead.
You may now start exploring some of the nearby rooms. The next room #1 contains a crate with standard ammunition and 15 rounds of gold ammunition. Exit this room and proceed to your left. Destroy a new door #2 using a knife or one of your primary weapons.
Once you're inside the room you should look around a bit to find supplies and a safe #1 with 44 rounds of gold ammunition. You can also check rooms found to the right if you have the time. Make sure to use a knife to destroy each door. One of the rooms will be a public toilet and you can examine a dead body here #2 to find a lot of ammunition.
Continue exploring the area and eventually you should be able to find a small entrance to the sewers #1. Use a ladder and don't forget to put on a gas mask. Proceed to your left and then turn right to leave the sewers #2. You can remove your mask here.
Once you're inside a new cave #1 you should notice that a new monster has appeared in the area. It's going to be almost identical to the creature you've fought not long ago #2. It would be possible to avoid this fight, however you wouldn't have a chance to find all the stashes. If you want to fight the creature you'll have to pay a lot of attention to not losing balance and falling. I would recommend that you keep using a shotgun. Notice that the creature will be using tunnels instead of vents, however the overall concept of the duel will remain the same.
There are two interesting areas inside this cave. You can perform a jump to a platform located in front of the entrance to the cave (that's where you've seen the monster for the first time). Locate a dead body here #1 to collect a medkit, standard ammunition and 18 rounds of gold ammunition. As for the other location, an upper area of the cave contains a crate #2 with a lot of standard ammo.
If you want to leave this area quickly (without fighting the monster) or if you've already explored the cave you'll have to drop down to a metal balcony (the one with a dead soldier) and choose a small tunnel located to your right #1. Eventually you should be able to find a door #2 and you'll have to interact with the left switch to unlock it.
Head forward and it shouldn't take too long until you've reached a train platform and rejoined with the rest of your team #1. Check your surroundings to find a new gas mask and then enter one of the metro cars #2.