Near the beginning you will come to a bigger yard with some doors, but only one door is unlocked. Go through it and there will be 2 enemies.
One will attack you and the other one fights against a friendly soldier. Ignore the First one an Kill the one, who attacks the soldier and than take care of the other enemy. If the The friendly soldier still lives he will sit down on the floor and
##Achievement unlocked##
Because of the camera you might not see the enemys directly after you enter the area- just walk forward and attack them as soon as possible.
After the soldier sits down you can talk to him by pressing "A", but for the Achievement it is not necessary...
There will be more places to earn this achievement - this is only the first one and a very easy one.