This is a pretty long portion of the DLC. It involves going to the library to get artifacts. Here's my top hints for survival....
1) The only gun you need is one you have - the pistol that shoots incendiary rounds.
2) Sell the other gun, and any other ammo, other than the ammo for the pistol.
2a) Trust me, you need no other weapon in this game!
3) Play on easy :)
Head to the surface and collect the first five items - they are all super easy to find. Loot every drawer and desk you see.
Now go back to the base and sell the artifacts. You should have enough money to buy the suit on the left that makes your filters last longer. Buy it. If you have any cash left, buy a couple more filters. Every time you go back, buy at least one filter. You don't need to buy any more rounds because (a) the pistol one-shots nosalis and (b) there's plenty of ammo lying around.
As you progress, new exits appear that allow you to get back to your base quicker.
Remember, if you shoot a nosalis, loot it. Don't shoot any other mutant, including especially the librarians. You will lose.
The level finishes when you get the map of the Metro and return to your base. You can find the map deep in the library in what appears to be a archive of some kind. You know you are close when you meet a second set of Librarians deep in the bowels of the library. The Librarian is sleeping, and wakes up. Just stay crouched and don't take your eyes off of him. When he jumps to the level above, go past him and take the ladder up. You'll see a chair in front of you, the vault is on the same wall as the chair. Watch out for the Librarian!!! Enter the vault/archive and the map is on the floor. Now return to the base and the cheev pops as you walk thru the door you use to make your checkpoint saves.