You can get this toward the beginning of the second mission after at the docks of Karnaca. When you leave the docks area you will find a wall of light, with two guards threatening a man (there is a map at the end of the pier where you first arrive; the wall of light is marked on the map). You need to kill or distract the guards before they push him into the wall. A crossbow bolt to the head of the one doing the threatening should allow the man to escape and the achievement to unlock.