Mario Tennis Open Hints for 3DS
It is recommended you have fast character with a lot of spin (Luma or Luma Suit was my choice)
Have Bowser be your opponent and choose the desert course. (Low bounce and slow speed helps)
Send him to the back of the court on one side and go for a power-drop shot (White Bomb-omb Circle) on opposite diagonal. He should be too slow to get to it if he is at the back at the time.