Simply start a new game on the level "Bad Blood" and proceed to the part of the level where you enter the base. Continue forward and stealth kill the first guy by two sets of boxes. Chase after one of the other guys (depends on how fast you are)and kill him. Now the last two guys will be aware of your presence, now shoot them or pin them with any weapon and proceed to the next room. Stab the guy that is walking to the right. Turn around and head to the other side. Stab the guy there. Now walk around carefully as two more soldiers come down a set a stairs. After you've killed the last guy eliminate the other two. If you want you can even head upstairs and continue, but its just quicker to stop after you've kill hose seven. If you get caught, its no big deal just pause and save your game to record the kills you got, then restart to your last checkpoint, and repeat a couple times to get your achievement. This does require a bit of a learning curve to get all of the soldiers, but after a few tries you'll get them all no problem. Don't forget you can check your progress on the main menu!
I made this video a while back in case you need more help.