To obtain this achievement, you must kill anyone who is running to call the cops who has a yellow phone above their heads. An easy way to do this is to find a room where there is only one exit. Doing this will cause anyone in the room, who is going to call the cops, to have to run right past you. Simply kill someone in the room and walk toward the exit. Once the person detects the dead body and knows its you, they will get a yellow phone above their head and begin to run. All you have to do is stab this person quick enough before the run away. Doing this 20 time will grant you the 25 gamerscore.