Wade through the water; [DOCUMENT - #2 CONFESSIONS OF A SOLARII (THE BEST JOB)] can be found on the desk straight ahead. You'll overhear two guards talking about Mathias. Go around the corner and turn the valve to stop the gas; this is poisonous (and inflammable) gas, and whenever Lara stands in or too close to it she'll take damage.
Move through the crack and collect the [SALVAGE] from the box. Look around the corner of the walkway to spot two guards moving an explosive barrel. As soon as they're both standing next to it, shoot it to instantly waste them both. A third comes running down the hall, so pop an arrow in his head, then move on.
The restroom to your right leads to another room with an [XP BOX] next to a burning barrel, and in the adjacent hall section you can crack open a [SALVAGE LOCKER]. Light your torch at the barrel, then head to the room left of the stairs. Inside are some [ARROWS] and a [NON-BELIEVER BANNER #1] that you can burn down; this looks like a wall poster.
First banner in the Mountain Base.
Upstairs a guard attacks you no matter how you approach, so quickly dispatch him. Move over the debris at the end of this hall and scramble up the wall to reach a generator room. [NON-BELIEVER BANNER #2] can be found on the wall to your immediate left as you enter. Hurl one of the lanterns against it to burn it down.
Head through the hole above the glass and turn the valve. Head back through the hole, grab one of the lanterns and hurl it through the hole to blow the area to smithereens. [250 XP]. Approach the hurt guard to take his [RIFLE]. [250 XP]. Don't miss the [NON-BELIEVER BANNER #3] in the corner of this room.
In the adjacent room you can find [RIFLE AMMO x2], a [GPS CACHE] in the corner, and [DOCUMENT - #1 WARTIME INTELLIGENCE (SCIENTIST: SECRET PROJECT)] on the desk.
You're ambushed as soon as you enter the next room. During the slow-mo, quickly dispatch one or two enemies, then hide behind the blue barrels and collect some [RIFLE AMMO]. You'll definitely want to use your rifle in this shootout, as there are many enemies. Quickly destroy the flashlights to get a better sight and dispatch the enemies closest to you, while jumping to the other side whenever molotov cocktails are hurled at you.
All collectibles in the Mountain Base area (except banner 1, see the other video).
When you've dispatched all enemies, search the place for [RIFLE AMMO] and a [GPS CACHE] on the right side of the area, behind some pipes. The adjacent room contains [SALVAGE], [HANDGUN AMMO] and the [NON-BELIEVER BANNER #4]. To reach the camp, head to the entrance of this room and use the fallen door to reach the upper walkway. You'll find some [ARROWS] next to the 'Map Room' base camp.
Head through the next hall and dispatch the last guard. Enter the control center to trigger a scene. [100 XP]. Search the place for [RIFLE AMMO], [DOCUMENT - #2 DIARIES OF A MADMAN (ALONE)] on the desk, [SALVAGE] and [ARROWS] near the control panels and [NON-BELIEVER BANNER #5] near the door you came through. Move through the cracks to the north to reach the next area. You can't actually get hurt during the part, so press onward.