F.E.A.R. Files

Playing either campaign on Extreme is tough, but the second one is definitively tougher since you'll face multiple Elites and sometimes multiple Elites at once. Doing this campaign on a lower difficulty first helps a lot.
Remember to always use slo-mo and take cover. Prioritise your weapons. Weapons like the Type-7, ASP, HV Penetrator, Type-12 are very useful and powerful and rank higher than the submachineguns. The shotgun can be handy at close range (like, in their faces!), but should also be avoided when range comes into play. The pistols are also very useful!

A good way to horde more ammo for powerful weapons (or every other 'dropped' weapon) is to NOT to already have the weapon in your inventory when you're picking it up.
Instead swap the power weapon with a different gun and then pick up the second one, then pick up the power weapon you first dropped.
This will give you MORE ammo than if you were to pick up the second one, while still having the first one in your inventory.
This is because the game gives you a bunch of ammo if you pick up a different gun, but less ammo if you already have the weapon in your inventory. Weapons YOU drop will keep their ammo!

Example:
If you swap your shotgun for the Penetrator, you'll get 75 rounds. If you pick up a second one you'll only get 10 more, giving you a total of 85.
But if you had swapped back for the shotgun and THEN picked up the second Penetrator, you would have had 2x 75 rounds, totalling 150 rounds!
NOTE: these must be weapons DROPPED by enemies, not the ones that are lying around for you to find!

Idk why the other guide recommends mostly garbage weapons and spends 80% on explaining the ammo glitch.

Anyways, PM isn't exactly difficult outside of the few Elite Encounters (the guys that keep insulting you and can climb through arenas).
Although the only ones that should you give you trouble are the very first one since you only have access to the Shotty by that point and the final one that has a Rifle/GL-duo immediately followed by a GL/Laser-duo. The laser in particular will annihilate your health at any range.
What I did was blitz to the upper part of the arena (holstering your weapon will give a significant speedboost) where the 2nd duo will spawn and shoot from above with the minigun so I could immediately kill the GL-guy of the 2nd wave once the other 2 were dead. I munched ALL of my meds doing this while beating it first try, but you'll have plenty ahead and backtracking is always an option.

Speaking of the Minigun, make sure to grab the first one from the Heli and keep it for the entire duration of the campaign. Somewhere in Interval 3 you'll come across another 450 shots - no clue where though, but in both my playthroughs I somehow walked over it so it's not exactly hard to miss if you're on the lookout for meds+armor. Thing is the easiest way to deal with the Elites - slowmo and baiting them into melee while unloading into their head kills them quicker than the Shotty.
Other than that, the G2 or VES will be your main-weapon for 90% of the campaign.
Shotty should be your 3rd - although you can temporarily drop it for the Lightning Arc if you know one (or multiple) of the heavies are coming up. Once Interval 7 rolls around, drop it for the repeating-cannon as you'll encounter a handful of enemies with them and the Shotty also becomes useless at this part.

Final boss is a pushover, only real threat is his final phase with the Repeating-cannon. But if you made it to this point you'll know how to deal with it.

tldr for the ammo-glitch: placed weapons in levels will always have low ammo. But if you pick up a weapon dropped by an enemy you currently DO NOT possess you'll recieve roughly 20% of the total ammo. Rinse repeat for max ammo after fights.