Metro 2033

There are three places with the machine gun, the first is Cursed, the second is Front Line and the Third is Trolley.

The kills accumulate as you progress the story, think of it that it saves your kills when the game saves, so if you load a chapter, like, Front Line, the kills you had in cursed are saved but not the ones you did in Front Line or Trolley.

In Cursed, you can use the machine gun to pick a spot where the mutants will attack, the mutants will not stop spawning until you blow up their point of entry but the problem here is two fold, the first, the machine gun only has 50 bullets, the second is that the other NPCs might steal your kills. I personally unlocked it here, took me around 20 minutes, you have to be patient and just aim at a single spot and quickly shoot when a mutant appears. The spot I picked was the left archway entrance from the left tunnel, close to where Khan is. I maybe killed 1 or 2 mutants every 1 or 2 minutes but I got it here.

If you run out of bullets in Cursed and still have not unlocked it, just proceed the game and get to Front Line.

In Front line, sneak into the machine gun on the other side of the bridge and use it to kill the Reds that are on the beginning of the stage.

You can kill five or six enemies this way.

If you still have not unlocked it, go ahead to Trolley where you man a machine gun on a railcar, here you can kill 8 or 10 more enemies if you are good and quick, it should unlock.

I wish you all best of luck!

You can do these achievements on the level Cursed.

Ninja & Sterling Effort

After Khan told you to blow up the tunnel you will see stairs, go up the middle stairs and farm there. You won't die on Easy difficulty.
For Ninja achievement you don't need to throw the knife, just use stab attack (right-click).

Heavy Metal

You can one shot nosalises on Ranger Easy. The mounted gun has 50 bullets and you only need to kill 15.

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