Walkthrough: Start off by going to your left. You shouldn't have any problems finding a body #1 and you can examine it to collect 5 rounds of gold ammunition. Choose a good weapon from your arsenal, because you're about to take part in a difficult duel. Head down #2 to a room where you've already seen one of the monsters.
You're going to be attacked by a large mutant here #1 #2. Sadly the beast is going to be very resistant to the gunfire, so it won't be easy to kill it. I would recommend using a revolver and you should always remember about aiming for the creature's head. Don't forget to stay on the move and to prevent the monster from moving too close, because several hits will be enough to kill you. Don't be fooled by seeing the beast escape, because it'll return here after a short while and the duel will be resumed.
Keep fighting until you've succeeded and then check your surroundings to collect ammunition from a dead body. Find a new door and destroy it #1. Get ready for a new monster encounter after making a few steps #2. You should know that you will be allowed to skip some of these fights. You can always try moving back slowly and waiting for the monster to leave the area. Naturally this won't apply to all duels, but you will be able to avoid some of them, allowing you to conserve ammunition and medical supplies. If you have to fight the monster mentioned above make sure to move back to the previous room, so you'll have more space to move.
You may continue exploring nearby corridors, finding a dead body near the window #1 among other interesting things. Use a knife to destroy a nearby door and be prepared for a new monster encounter #2. I would recommend using a shotgun against this mutant (if it attacks you), because you'll be fighting in a small room.
Make sure to collect a medkit along the way and then crawl through a large hole found in one of the walls #1. Destroy a new door to find yourself standing inside a public toilet. Take supplies from a new body and check the second room to locate 5 rounds of gold ammunition (on the ground to your right) #2.
Return to the previous room and walk through a much larger hole #1. Approach a smaller hole located directly in front of you where you'll be delivered a cheap scare. Don't worry, because you're not in any danger here. Continue exploring nearby corridors and you should soon get to a staircase leading to the upper floor #2.
Be careful here, because a new mutant may appear near your position #1. You'll have to watch out for holes found in the ground while exploring this floor #2. Thankfully even if you make a mistake and fall through the hole you'll end up in a previously explored area and you'll only have to use the staircase again to resume the mission.
Be on a lookout for a much smaller hole #1 and make sure to use it to continue your mission. Take your time to examine a body found in the area, collecting ammunition and gas mask filters. You may proceed towards a metal gate #2.
Don't be surprised to encounter a new mutant #1. You may able to skip this fight, however you would have to immediately proceed towards a nearby hole #2. If you're too late use the methods explained earlier to defeat the creature.
A nearby door is going to be destroyed by a new monster, however this mutant should leave the area without attacking you. Make sure to check a newly explored room to find a body #1 and 5 rounds of gold ammunition. You can return to the upper floor easily and without having to leave this room #2.
Return to the area where you've jumped down through a large hole, but this time remain on the upper floor #1 and continue moving forward. You'll soon watch an interesting cut-scene and you'll regain control inside a destroyed elevator. Take a medkit and proceed forward, ignoring a new monster along the way. Open a new door #2 and this mission will come to an end.