Fairly easy achievement, but what you need to know is that you need to use LOW draw weight on the bow. Go to the screen where you change your bow ammo, hold
button. You can also change your draw weight as indicated in the upper right hand. Its actually a really easy thing to miss and I am sure plenty of people played through the entire game without realizing you can draw you bow differently. It needs to be on low draw weight. Then shoot some poor dude in the knee. (Also, of course, use normal arrows)
But here is the tricky thing. You can not kill him, you need to wound. So try and just flick the arrow, pull the bowstring back a touch, pretty much just a quick pull on the trigger, so it's not a full pull.