For this achievement you need to kill 5 enemies with the manhack. For this you will need the gravity gun and the best chapter to start this is Nova Prospect. While playing thru nova prospect into the next part entanglement you will have an opportunity to get this achievement. Simply grab the manhack with the gravity gun and shoot it at an enemy. If you don't get enough manhack kills here you will be able to achieve more in Chapter 10 AntiCitizen One when entering the tunnel.
Also note that you do not need to get this achievement in one play thru it will carry over if you have completed the game already.
Another note: You cannot reload save or die to get more kills they will not count if you are trying this method. ie: Getting a kill then dying and getting a kill then dying.