Play through the beginning cutscenes for several minutes, including riding the elevator with Anna and Miller, getting captured, then regrouping with your squad.
Once you're in control of Artyom, you'll be ambushed by Cannibals while waiting for the elevator. Kill as many as you can, then head into the elevator. While you're riding the elevator, search the ammo boxes to replenish some of your ammo.
Once the elevator stops, head off then go forward. Once you pass through the security gate, look immediately to your left, and on the table is Diary 26/71 (Diary Page 1). To your right, interact with the yellow switch to turn off the lights, then head back through the security gate and on your right will be a yellow power box. Once you've interacted with it, turn back around and you can proceed through the door that's now partially open.
Once you're on the other side, there will be a bunch of cannibals charging at you, so kill them all. Make your way through to a red door, bust it down, and you'll be in a freezer room full of more cannibals. Kill them and progress forward until you can interact with some shelves to squeeze through them.
Head to the control panel on the wall ahead to turn on the lights, then interact with the floor immediately to the left of it to crawl under some crates. On the other side, immediately in front of you on the right target board is Postcard #9 (Postcard #8 is found later on in the mission). Immediately to the left of this is Diary 27/71 (Diary Page 2).
Progress a little further and eventually, a pipe will burst and kill a cannibal with steam. Rather than heading through this deadly steam, climb up to your right and head to the path above through the vents. Head straight through it and drop to the other side, killing the cannibals here. Continue forward, killing more cannibals, and interact with the door at the end.
On the other side, you'll be in a tunnel with red flashing lights and more cannibals to kill. Head to the end but don't interact with the pipe just yet. To the right of the pipe, climb up to the crate and drop down to the other side. On the right side, head into the bedroom, and resting on the crate is Diary 28/71 (Diary Page 3).
Now keep progressing forward, taking out a handful of cannibals. Stay to the right side, and you'll come across a switch on the wall. Interact with it to open the door to your right.
Head in this room, and there will be a few cannibals and an armored enemy with a minigun. It will take a decent amount of ammo to kill him, so make sure you're fully loaded and maybe use a few Molotov cocktails to kill him (direct impacts with Molotov cocktails against armored enemies do a lot of damage).
Once he's dead, go over and pick up the minigun, which is called the Gatling. Head into the room where the armored enemy came from, and make sure to kill at least one of the enemies here with the Gatling (this should be your 7/10 weapon kill towards the "Professional" achievement).
In this area, head into the cell on the right that's marked with an orange skull and crossbones, and on the tray next to a hatchet is Diary 29/71 (Officer's Diary, 1). Continue forward and you'll be reunited with Anna after a small cutscene.
Once in control of Artyom, look at the doors directly behind the chair where Anna was at, and hanging on the wall to the left of the doors is Postcard #8. Then, head through these doors and advance down this hallway. At the end of the hallway, on a barrel on the left, is Diary 30/71 (Officer's Diary, 2). Head in the elevator and you'll be reunited with Miller. After some chatter, you'll all proceed through a door.
Follow your crew forward, but don't head into the elevator just yet. A few steps in front of the elevator are a couple of red fuel cans next to a crate. On top of this crate is Diary 31/71 (Officer's Diary, 3). Head into the elevator and ride it to the top, killing cannibals as you ascend. At the top, you'll head onto the trolley and a horde of cannibals will swarm after you. Pepper them with whatever weapons you have, and you'll likely have enough kills at this point where you'll unlock:
If not, you'll definitely obtain it in a future playthrough.
Play through the ending cutscene, completing the level and unlocking:
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