Velvet Assassin

There are 2 german soldiers sleeping in small huts. As you wander around you will see them as they are on your path. You need to use the knife to kill each soldier and the achievement will unlock once you kill the 2nd guy. The huts are quite obvious and there both on your right hand side as you go though the level, you will be required to walk past them at some point. The first one is near the fence you jump over after turning off the power with loads of shipping crates. The 2nd is near the safe where you pick up the secret documents.

This image marks their locations.