Load up the level LOST TUNNELS
. Follow your comrade, Bourbon until he will tell you about a tripwire. After you jump said wire, Bourbon will tell you about some bandits that have barricaded themselves and if you can take them out. This is a good spot to boost this achievement.
You can kill the first bandit who is easy to stealth kill without alerting the other bandits. Trow a knife at him to kill him. After you've killed him pick your knife back up from his body. Now you can kill some more bandits or reload checkpoint. Reloading checkpoints will not reset your progress! Keep reloading until you get the achievement.
Other achievements that you can also boost here: