This solution is the same as the one that Neatherblade's posted, apart from mine is in much greater detail, i noticed that his solution had received quite a few negative votes so thought i would make it much more understandable. A quick way that I obtained this achievement was to run through the first mission of Level 1.
First of all, load up a new game, first level, nightmare difficulty obviously. Note that: The enemies keep spawning so i would advise to do what I say in this guide and run, unless you wish to take them all on, bad choice in my eyes.
Then once the game has started, immediately turn left towards the stairs, there will be a short box at either side at the bottom of the stairs, look on the right side of the right box and there is a red switch panel, press the switch with 'A' (beware of the enemy standing behind the box) then quickly turn around and run straight forward back to where you started and jump out of the window. Or if you want to make the rest of the level easier, kill the enemy behind the box and he will drop a shotgun, which you may need later on and is more powerful than the pistol you start off with. LT is to sprint by the way. If you are not quick enough the wall will rise and then you will have to press the button again.
Once outside, sprint to the armor in the middle of the acid lake. Then turn right and head down the stairway, beware of the 2 enemies at the bottom, i chose to kill them but you may be able to run past them? save the game before you enter the stairs just in case you die. At the end of the corridor is a door, open it and quickly run right because there is a few enemies to the left and they will instantly kill you.
There should be a door in front of you, with enemies on the other side, you can either run straight through there, or take the enemies out (i took them out) and then open the door at the end of that small room. Once in the room, there will be an enemy which you have to kill, kill him and then press the button and the game will end, then press continue and the achievement should pop up.
This will also unlock the NEOPHYTE achievement as well, which is for completing any level. That is if you don't already have it.
Hope this helped a few people, this was the way that i got the achievement, it is much easier than trying to go through the full map killing all the enemies. Please leave a positive vote if this helped any of you guys. Thank you.