this can be done anytime over the course of the game, though i tend to go for head shots for the extra experience. if you don't want to "waste" your kills, then read on.
a good place to get this, if you haven't already is the part where you are in a room with 5 guys and one on the ground. they say "leave him there". you can take out the guy on the ground and 4 others with leg shots, earning the achievement, then leaving one alive, pause and reload checkpoint. now you can use the gun here and get another achievement: