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. Make sure to move close to the enemy and wait for some text to appear where it says to either knock-out or kill the enemy. Make sure you kill him. It has been reported that normal knifing does not make progress towards this achievement and that you have to take out the enemies when the text appears.
Now you can kill some more bandits or reload checkpoint. Reloading checkpoints will not reset your progress! Keep reloading until you get the achievement.
This place is also great to boost these achievements: