Firstly select survival mode when starting a new game. The main differences between survival and spartan mode are:
- Limited resources ( I however didn't notice much difference with regards to this)
- More intelligent A.I. (I noticed some stealth sections became a bit more difficult)
- Maximum of 20 minutes gasmask filters (on spartan I believe you can hold up to 30 minutes worth)
When playing through try to search as much as possible for ammo/supplies. Also aim to preserve ammo by going for headshots on enemies or avoiding combat entirely.
Below is an outline of levels with tips on how you can avoid combat and preserve your resources for mandatory battles. There are SPOILERS.
*** Spoiler - click to reveal ***
Intro: N/A
Tower: N/A
Hunter: N/A
Exhibition: N/A
Chase: Use the shotgun as much as possible as it has unlimited ammo.
Riga: N/A
Lost tunnels: Let your partner kill most of the mutants; use ammo if you are surrounded.
Market: N/A
Dead City: There is one mandatory fight when you reunite with your partner. You can run through the level quite easily. You can also let him kill most the enemies in that battle.
Dry: You can avoid combat on this stealth level. If you get spotted it is possible to sprint to the door where your partner is being held (if playing on normal with medkits at hand).
Ghosts: At the end there is a mandatory battle, let Khan kill lots of enemies. When the anomaly appears don't kill any more enemies.
Cursed: Firstly run up the stairs to the right to set the dynamite. Next run to the right tunnel to get the gas tanks. Kill the mutants that appear as they can hinder you. Next run to the left tunnel and plant the bomb. In my game I could run straight to plant the bomb past the mutants. After setting it up the enemies disappeared.
Armory: N/A
Front line: You can get through the entire level without using any ammo. Ideally follow the other video guides on how to get through without killing anyone (a visual guide is more helpful).
Trolley combat: N/A
Depot: You can save ammo in the first section by killing the guard on the tracks, doing this allows you cart to go straight past the stop where many enemies are. Near the end use a shotgun for 1-2 hit kills on the mutants that board the trolley.
Defence: Let your team mates kill most the mutants, though you probably get swarmed quickly in this section at the beginning. When continuing on your own run past all the lurkers picking up ammo if needed. In both sections of the station you need to sprint straight - left - straight again. Eventually you find a child, you can sprint straight to the end of the level avoiding all combat.
Outpost: I don't know whether it's possible to stealth through the entirety of this difficult level. In the first section I climbed the ladder to the right killed the enemies on the middle and top level. Reach the crane, climbed up to the floor and sneak past the Nazi enemies to the staircase. On the bottom floor I killed all the enemies. When you reach outside you may be able to sneak around the enemies but it never worked for me. You will reach a descending staircase at the end.
Black Station: Follow video guides for the "merciful" achievement to avoid all combat. You can finish the level in a few minutes using that route.
Polis: N/A
Alley: Sprint past all the enemies at the beginning until you reach the front of the library. In this mandatory fight you are on your own for the first half until two comrades arrive to assist you. When they arrive aim to save your ammo by letting them kill most the enemies.
Depository: There is 1 librarian you must kill at the beginning of this tense level. Ideally use a shotgun and sticky grenades. Sneak through the broken door, continue until you reach a bathroom, head through until you find hold in the wall you must go through. After this turn around the run and sprint to the second hole in the floor and drop down. Go to the door across the hall where a librarian will appear, navigate around it and head up the broken shelf in the room the enemy appeared from. You will now be back on the higher floor, head to the other side of the floor and sprint to the elevator. After the cutscene a librarian with appear at the end of the room you can kill it or sprint straight to the door to end the level.
Archives: You can avoid all combat on this level. Firstly sneak past the 2 librarians until you reach a staircase. When you reach the bottom head right up the slope, across the boards to find another staircase. Jump across to a ledge when you reach the top and continue going through until you can drop down. When you reach the bottom floor head left - right - left - right sneaking past the enemies. Ascend the staircase at the end, head right then across the wooden planks. Descend the staircase and drop down. Next sneak to the right to avoid the enemy, continue forward and go up the stairs. Lastly head to the last door on the left to finish the level.
Church: N/A
Dark Star: Let your team mates kill most the enemies and use the flamethrower in short bursts when the mutants climb the trolley.
Cave: Let your many team mates kill most the enemies, I'm not sure whether the friendly A.I. has unlimited health though. The "Bigun" shotgun works well killing the enemies in 1 or 2 shots. When you reach the spider enemies run past them to the end of the level.
D6: Kill the amoebas with the bigun or hellbreath for 1 shot kills. You can also let your partner kill them.
Tower: You can use as much ammo as you want on the final level. Once you start ascending the tower you only need some ammo to kill 1 demon in a cutscene.