There are four turrets to use - 2 on either side of the map. The first one is at your start location. Jump straight on it and in a few moments the first Wraith will come down. Shoot it.
After that, move up the level on the same side (right hand side). You start with the camouflage ability so use it to get to the next turret easily. Once you have used this turret, activate your camouflage (double tap your Spartan) and go over to the left side of the map. There are two more turrets. Make sure you clear the grunts and wait for a Wraith.
The achievement will pop before you finish the level so after you take out a Wraith on the fourth turret it's all yours.