I unlocked this as I played from chapter 4 - 6 on easy difficulty working on my survivor trophy.
I was continually trying to get a streak together as these chapters give you a lot of enemies that are reasonable easy to kill using this method.
I selected easy difficulty and then worked through the chapter weakening guys using one shot from the hip with a pistol and then the one punch kill.
I found almost all enemies were killable with one punch after one shot from the hip and even managed to get one of the heavily armored shot gun guys by shooting his helmet off first.