To get the "Hail Mary" achievement on Read Dead Redemption, it's going to be quite a challenge. This is almost impossible to do while standing on flat ground, but it is possible.
The best way to do this is to head to Pike's Basin. Start the Hideout mission in Public or Private Freeroam. Then die so you start at the top of the canyon. Once you restart, you can then head to the far right objective and stand at the top of the canyon. You should have a pretty good view of the enemies below. Take aim, then aim above them while throwing a tomahawk.
If you keep at this, you should kill one of the enemies below and earn the "Hail Mary" achievement. You can also do this with a friend, but it could be far more time consuming.