1. You will need to get the Knife item. You will find it very early in the game. It's impossible to miss it. It's a melee weapon that has infinite durability but only 0.2 damage (the shovel has 1 and it already is crap). But the truth is, this is the best item.

2. Enemies can be backstabbed and instantly killed with a charged attack while they have not seen you. As far as i can see sound doesnt matter in this game, only line of sight does, which itself is BROKEN beyond belief (God knows how many times i was spotted through objects or while hiding after breaking line of sight because of German troops magic...), but regardless;

3. Find an enemy that is walking opposite to you, so, with his back facing you. Approach him normally or even running in my experience it doesn't matter. If you want to be safe wait until he stops but you can also do this while they are walking.

4. When you're very close to the enemy (but dont touch them, i think this aggro them) hold RIGHT CLICK to charge up the knife attack and WAIT until it reaches the maximum charge (so, knife all the way up, try the animation many times to see exactly what is the max point) and then release the attack with your sight ON the enemy.

5. The knife has more range than it appears so even if the enemy is 2-3 steps ahead you WILL hit him. If this is done correctly this can instant kill normal enemies including shovel guys, rifle guys and even the masked medium armor guys. You will spend no ammo and no durability.

6. If there are many enemies dont worry: an enemy facing back will take 2-3 seconds to aggro together with other ones that aggro you so you actually have time to go, backstab them, and then run away to the nearest door to reset the enemy alert. Or wait until they isolate a bit, your choice, if possible. Go back and forth in doors also to reset enemy spawns, maybe one is facing towards you? Ok keep trying the door until it faces back next time so you can stealth kill them.

7. And now you will wonder: But what about the rats? Well the trick with them is simple: equip Knife and spam left click, doesnt need charging, it will kill them in one hit before they can attack you. And you can even spam it if out of stamina. Works for me, I do it all the time. However this MAY not work if the amount of rats is high, in this case i suggest, if the body with rats is near a door, to reset the rats then rush the body and spam left click before the rats have time to aggro which takes about 2 seconds. But burn bodies in KEY areas like where you gonna pass many times.

MAY NOT WORK ON SPECIAL ENEMIES e.g. armored brute with axe etc. didn't have a chance to test it yet.

Im playing on the hardest setting and so far i have shot very few times, cleared entire areas just with this method. It may be a pain in the butt, but it's efficient and given how depressing the economy of this game is, you will need it.