You can get this achievement by playing through a tournament in World Tour (it could be any tournament, but I did it in World Stage 3)... alone. All you need is a second controller.
Play through the entire competition until you reach the final. In the final, play alone and make sure you gain a comfortable lead. Obviously playing on Easy (Bronze) difficulty would make things easier.
Now, with about 10 seconds to go, pause, plug in your second controller, and go to "Select Sides" on the menu. Push the second controller to the same team as you, and then see out the last few seconds.
The achievement will unlock after the prize-giving cut scene!