This achievement requires you to kill 15 enemy aliens with the turret mounted on a Humvee.
At the start of the second level, you will be asked to mount a turret on a Humvee to provide cover. At my best count, there are approximately 20 enemies that spawn here, so kill 15 of them. There is a second turret sequence at the end of the game too, if you unlikely miss it this first time.
The achievement should poop after the 15th turret kill.