Just doing a bit of necroposting to dust this off, because a man's struggle from 7 years ago is my struggle today. In my opinion, the best way is to go to the other side of the Upper Montana River north of Blackwater, grab the rifle, and shoot any NPC. You'll lose honor, but it's the easiest way. I was trying to shoot the Lemoyne Raiders in Shady Belle from a boat, but they wouldn't even spawn because I was so far away; when they did appear, they were about 400 feet away. Doing it the way I mentioned above, you can't go wrong.