Update: several people in the comments have confirmed izret102's advice that you can FAIL the final round and still get this, as long as you are under par time.
Original Solution: This can be done on any difficulty, so set up a Speed Rescue game on easy. You'll have to save every Graydian AND achieve a time under par in each room. A time under par is when it comes up saying "Lightning Speed!". If you have any problems, restart the stage, and you will get to play the same rooms again and can perfect the route.