Walkthrough: You should act quickly as soon as this mission has started, because you won't have much time to collect items from this section of the catacombs. The bodies found here contain a lot of standard ammunition but you should be more interested in collectibles - find 4 rounds of gold ammunition #1 and 5 rounds of gold ammunition #2.
You will witness visions featuring Bourbon in the meantime #1. Ignore the first and the second vision. The third vision is a different thing - you will have to complete it if you want to survive. Thankfully all you have to do is run towards your colleague (he's standing close to the gate) #2. Wait for the game to play a new cut-scene.
Start exploring a new area of the catacombs and be on a lookout for a passageway found to your right - it will lead you to a large round chamber #1. Use the balconies and ledges found here to get to the other side of the room safely. Find a body there #2 and examine it to find a unique revolver, standard ammunition and 3 rounds of gold ammunition.
Return to the main corridor and keep heading forward. Be careful when passing by large holes and don't forget to examine a new body #1 to find 20 rounds of gold ammunition. You can now choose between remaining on this level and choosing a nearby hole in the ground #2.
If you've decided to jump down the hole you will have to put on a gas mask to stay alive. Make sure to examine the surrounding area carefully to find 4 rounds of gold ammunition #1. Head down again and this time find a corpse containing gas mask filters and a lot of standard ammunition #2. You can go back up using a nearby ladder.
If you've decided to remain on the upper level you will have to keep following Bourbon #1. It shouldn't take long until you come across a large gate #2 (this will also occur after you've used the ladder). Wait for the conversation between Bourbon and the guards to end.
Make sure to choose a good weapon from your arsenal, because you're about to be attacked by a large group of monsters #1. Prevent the creatures from moving too close and be ready to solve mini-games in order to avoid certain death #2. Keep fighting until all the beasts have been defeated and wait for the gate to be raised. Entering the base will end this mission.