This achievement is easily miss-able. In Chapter 6 when you meet back up with Joseph and get the Sniper rifle, if you go right, there is a ladder you can climb. Once on the upper level when you move to the bridge. Lure the haunted towards you and they will knock Joseph down so that he will be clinging to the bridge like a crazy ex girlfriend. Simply kill the haunted and save him and let that crazy ex back into your life. You should take out the guy shooting bolts at you first before attempting this. The bolts won't knock him down, just damage you.