You will get this achievement for unlocking the seventh stealth perk.
For this perk you have to perform 5 stealth kills on Commanders. Simply sneak up behind them and press the right stick to perform a stealth kill.
If you want to do this quick you can always reload checkpoints and kill the same Commander over and over again.
You can track your progress on the perks-screen if you are not sure how many you have left to do.