Walkthrough: You should start exploring this location by entering one of the rooms located to your left #1. Don't worry, because you're not in any hurry and your allies won't do anything important without you. Check this room to find 35 rounds of gold ammunition #2 (five clips), as well as standard ammunition and gas mask filters.
Return to your allies and continue moving forward. Once you're inside a new room #1 you should listen to a conversation and then explore a smaller room found to the left. Turn left after you've entered this room and collect 10 rounds of gold ammunition from the ground #2.
Go back to your allies again. You'll have to interact with four switches found inside this room #1. Two switches are located to the left of the large gate and the remaining two are to the right. Wait for the gate to be raised #2.
Head down and get ready to take part in the first monster battle #1 #2. Your allies should be able to handle most of the beasts, however you must know that they're not immortal and that the death of one of your companions will be equal to failing the mission. I would recommend that you only assist them, especially if you notice someone from your team surrounded by a larger group of monsters. You should also watch out for additional mutants which may appear in the area where you've came from.
Keep heading forward and expect to encounter more beasts along the way #1. You should still be a passive member of the team, however you can't just stand in the back and do literally nothing. One of the monsters will soon kill your ally #2 and you can't prevent it from happening. Walk through the next gate to end this mission.