An extremely easy spot for this trophy is in "Chapter 8: Under Siege ; Scene: A One Man Army". Here you have to use a rocket launcher to kill waves of approaching enemies. Immediately after you take control of the rocket launcher, aim to the bottom right and there will be a group of 3 enemies behind a broken fence. Shoot them and the trophy will pop.
If you don't get it on your first try you can either restart the checkpoint or wait for other groups of enemies. Just wait until 3 or more enemies are close together and then take your shot. There are plenty of opportunities for this when you have the rocket launcher.