This achievement should be unlocked over natural progression throughout the game. I personally unlocked it after the part in Chapter 12, where you operate the Jeep mounted machine gun. This section is also a great section to farm kills for the achievement. You get a checkpoint after retrieving the medicine from the ambulance. Then kill the machine gunner and take over the gun, once the enemies stop, do not get on the bus but rather reload the last checkpoint. Once you get enough kills the achievement will pop.