Contrary to what everyone else has said there is a much simpler way of getting this achievement without needing to ever go into combat. First, make sure you have Dexterous styles level 4, this will allow you to select specific body parts to target while aiming. Go to Murgo in Bowerstone, now turn your safety off, press the RB to do so. You can now aim freely at Murgo, hold Y, hold LT, aim down and release Y for a groin shot. The catch is Murgo will never die, will not move from the spot where he is standing and will take the groin shots without putting up a fight, also you are usually far enough away to not alarm anyone of your actions meaning no combat necessary.
*You need the See the Future DLC to unlock this achievement.