Pretty easy and self-explanatory. This one will unlock very early in the game and is of course unmissable. If you follow the written walkthrough on TA you will find yourself playing the first level over and over again for 1-2 hours to max the stats of two of your cops. Do so and keep killing the last enemies in the stage (those in the last room) and you will get this achievement before you are done with the grinding.
If you don't follow the guide it will most likey unlock in the middle of your first playthrough, depending on your skill and your killing/busting rate.
Hope this helps.
Any imoprovements? Please let me know in the comments below