Took about a dozen attempts, couldn't find any clear instructions in the manual or game guide. I aimed with the 1º line right over the guys chest. I aimed using keys instead of the mouse. Finally got the sucker. I was getting sick of Geralt saying 3 degrees or 2.5 degrees regardless of where I aimed...
Got it on a second try, while looking at through the scope you should be an almost bottom of the box square. I didn't think of any achievement when I did and didn't think much at the time. I just wanted to nail it the first time, but couldn't do it and had it on the next one.
I got this on a 6th or 7th try. I finaly gave up trying to hit the guy with the plume and aimed for the second guy on that balcony and it worked. Tryed it a few more times and hit the plume guy all the time.
All you need to do is to try and alline the middle of line II with the second guy just above the edge of the wall. He says 1.5 degrees and bolt hits the wall killing bouth of em.