This can be earned midway through the mission.
you will come to a point where you will place a bomb on a door. A timer will begin to count down, and a scoreboard will pop up on the screen with whoever you're playing as on one side and the AI on the other side. Your goal is to outscore whoever you're not playing as by killing the most BATs. This is very easy to do if you're playing on an easy difficulty. I did this on the hardest difficulty and still managed to win by eight points. It's a very easy achievement.