They key to this achievement is deadeye. Just try to take every shot you take with dead eye(click the right stick while aiming with the LT). Towards the second half of the game, you have plenty of money, so just keep stocked up on moonshine, tobacco, and snake oil to refill your dead eye meter when it depletes. Also note that once you have reached level 10 in the sharpshooter challenge, your dead eye meter gets replenished by killing enemies, so it becomes much easier to have dead eye available(note it only replenishes for kills gotten WITHOUT deadeye)