Don't stay on a horse. Stay on foot and shoot with your pistols till he runs up the side of the ridge. When you see him do this, you will notice he is healing. Shoot him TWICE with the sharpshooter rifle and he will charge back down at you. As soon as he gets kinda close, go into quickdraw and shoot him as much as you can. with your pistols. After QD is used, quickly shift to your shotgun and shoot him twice. Sometimes you can kill him in one pass if you don't miss too much. No more than two passes ever. If you run out of QD when he charges you from the ridge, he always runs through the wreckage at the safe to heal. Long as you run after him, he won't stop to heal there and will charge you and head back to the ridge. You can also get quite a few shotgun blast on him here. That's really all there is to beating him.