I can't take credit for this as I got this information from a guide for the the 360 version of the game, and I'm surprised no one mentioned it here in the Director's Cut as this is probably the easiest way to get this achievement. So on the 360 version of the game JullianGG has a guide for this achievement, and FOV1863 left a comment on that which Jullian updated to be in the solution itself and this is what got me the achievement on the first try rather than dealing with the seemingly RNG nature of just hitting them until it works or running up to them and smacking them from behind. You'll first need to wait until you get the revolver. Then when you run into a Shadow aim for their leg and shoot them, this will cause them to hunch over and then hitting them with a melee attack will take their head off. I can't say if there is a chance that it won't, but to me it seems the success rate must be high at least as I got this to work on the first try. I made a video to demonstrate if you would like a visual as well;