Quite simple way to get this is by setting up a offline multiplayer match with a second controller. Choose a Combat Race on Canyon 1 and leave everything else and then start the race. When the race starts, with the second controller just turn around and fly over the weapon pad and if you don't get a rocket straight away just randomly fire the weapon and go back over the pad. Now when you get the rocket go up behind the first controller's ship and fire the rocket, as soon as the rocket is attached quickly on the first controller hold X and A together and then after 2-3 seconds release X and you should speed of with the rocket somewhere behind you. This takes some timing so you might not get it first time around.