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.
Now take note that you have to kill 15 enemies in a row without breaking stealth which kan be annoying. However there is cheese way which makes this a lot easier. You can kill the first bandit who is easy to stealth kill without alerting the other bandits. Trow a knife at him of walk up to him and press
when prompted on screen to kill him. After you've killed him, reload checkpoint. Reloading checkpoints will not reset your progress! Keep reloading until you get the achievement.
Some other achievements that are also perfect for this level: