This achievement can be won anytime in the game that you kill someone from behind cover (press LB when up against a wall, then press forward on the stick to pop out and shoot) , but your first real opportunity to get it is when you're assaulting the farm in the second level.
As soon as you clear the farmhouse you come to an MG. It has infinite ammo, and approximately 20 guys come running towards you. Turn it on, and mow em down. Your teammates aren't great shots, so you should be able to get at least 10-15 kills from this location.
If you got most of your squads kills so far in the game, you should also unlock a few other achievements at this location as well.