This achievement is not as hard as it sounds but it can be easily missed. After you go down the stairs from the tower, stick with the crossbow, as it is much harder to miss the target. Then kill the guy on the right using his spade/axe.
After you go into their base:
- Kill the guy closest to you walking
- Then, kill the 2 guys on the truck on the far right
- Then, kill the 2 guys on the truck in the garage (Far left)
Once you walk on:
- Kill the 2 guys on the left (recommend guy on right)
- Then, kill guy on roof to the right
- Then, kill the foot soldier by the truck on the right
- Then, kill the guy inside the building on the computer (to the left of the last kill)
Once you walk on, wait until the soldier is NOT facing you. Then, kill him and the achievement should pop up.
IMPORTANT: If the guy is hit but not killed and he is about to point his gun at you, this MAY AFFECT the overall achievement progress. Make sure that they are definitely killed first time.