I had so much trouble getting this achievement so I'm posting a small guide on how I got it. I played through this game 3 times and I didn't get this achievement (not that this was a bad thing, I got a couple more achievements while I did those runs. And the game is awesome). Only when i checked the Metro 2033 wiki i found out the biggest criteria on getting this achievement: You have to use firearms. So using a Helsing or the Volt gun will not get you the achievement. The revolver, shotguns and the assault rifles are the guns you need.

So, this is how i got it: I did this on the level Front-line, but i did some preparation in the Armory, before going there. The recommended difficulty is Ranger Easy.

At Armory, buy a silenced VSV, preferably with a scope. If you're short on Military Grade Ammo, go back to Andrew the Blacksmiths house and pick up whichever gun you don't want to keep and go sell it by buying the cheapest shotgun(the 2 barreled one), then come back and pick up the one you like. You will need at'least 30 MGA to spare for the next part.

If you want and have the money, you can also buy the stealth suit that is on the opposite side of the weapons booth. I bought it, but if you're confident in your sneaking skills you might not need it. I sneaked through it once without the suit and wasn't spotted.

After that hop on the hole in the ground, then on your way to the Front-Line.
The main thing you want to do is to get those headshots.Dirty ammo on the VSV can pierce Welder helmets, but not Combat helmets. If you're facing soldiers with combat helmets, try to get as close as possible, then aim at their exposed back of their heads, then fire. If you have plenty of Military Grade Ammo you can use those to pierce combat helmets and also soldiers with gas-mask's on.
Two examples of welder helmets.

I got my last required headshot at the beginning of the facists position, so if you're having trouble killing the Red's on their positions frontline, then you can skip them and kill some of the fascist at the end of their position to make up for lost headshots.

And thats that. You should be able to get the achievement with these tips. Good luck and lets hope that we will see Last Light released.