You can get this trophy at the start of Chapter 9.
Right at the start is a tunnel with a walkway near the ceiling. Kill the enemies on the ground and up top. Take the ladder on the right side to the walkway, and grab the rocket launcher there.
Now you need to find a single enemy by his lonesome. If you exit the tunnel, a tank will show up so take out the gunner. Kill the enemies nearby but try to leave 1. Use L1 to shoot 3 rockets at once, if you kill him the trophy is yours.
If you don't get it, you have another 3 rockets so you can try again, and you can also restart the checkpoint and grab the Hadra to try again.