The best place to get this trophy is in the Stockyards chapter. Rocks can be obtained by jumping up to a rock sack, which is usually hanging from a platform. The first time you can grab some rocks there will be some bats nearby. Simply hold rightstick to aim a rock and throw it with R1. Aim it towards the group of bats and with good aim, you will kill one and the trophy will pop.
credits: PlaystationTrophies