Call of Juarez: Bound in Blood

In chapter VI there are tons of horses at all important places while you are doing the three side-quests. I recommend getting this achievement here, because if you get a horse shot away under you, you can get a new one right away.

Thus take a rifle, approach the foes carefully and perhaps semi-hide behind a rock or a wall. Then take them out from a distance.

As the achievement text states, it's just a total of five enemies you need. Take your time and don't get too close, else they kill your horse too fast.

This can also be done earlier in the game when you have to get to the family home. After despatching a few enemy near a gatling gun there is a few horses (3) you can see. Get on one and make your way to the home. On approach there will be 3 enemy outside the home entrance, shoot them from afar then as you enter the home you will see more enemy coming down the driveway. Try to shoot 2 other ones to get the achievement.

This is where I got the achievement.