You will first encounter enemies with rocket launchers in the Weapon X facility level. To reflect a projectile press
just before it hits you. If you reflect a projectile it will instantly kill the enemy that fired it. During one playthrough you encounter approximately 25 enemies with a rocket launcher. So if you want to get this achievement on your first playthrough, make sure to kill all enemies with rocket launcher by reflecting the rockets. If you don't get this achievement in one playthrough, load up level one, checkpoint Light at the End of the Tunnel. There are two Grendadiers right after the start, and one just before exiting the tunnel.