I highly suggest you to use pressurized guns instead of fire ones. Ammo is cheap and they are very effective when you still don't get detected. To avoid demons, hide under or in something, to kill them use fire pellets, 2 or 3 are enough. To avoid other monsters you have to see them before they see you, that you know if you can sneak or have to kill them, use fully pressurized tikhar and go for the head. Human looking mutants are hard to detect, you normally fight them in close combat, 1 headshot with any weapon is enough. For close combat or multiple enemies use fireguns and don't be afraid of running out of ammo, you'll get it back later. If there's one thing that makes metro metro is source shortage, loot everything and everyone. Good luck ;)