Rocks are only found late in the game in Sorrow's Pass or in one of the areas once inside Mount Horu. They are used in puzzles that require players to block an elemental hazard. When you come across a rock you'll have to use your bash and aim down which throws the rock straight up. After the bash, you'll land on the ground, quickly jump and repeat the bash another 4 times making sure the rock doesn't touch the ground between throws.
It should also be noted that although some rocks cannot be moved once used to block a hazard, some of them are. To move them you'll want to go up them and hold the trigger in order to grab the rock and then move it back to a safe area before trying the achievement.