The Litwick Challenge is to a Shiny Hunt, in the Celestial Tower on the 3rd floor.
Here are the rules Sapphire made:
1) You MUST get it from Celestial Tower and on the 3rd floor, where the nurse is so you can heal your Pokemon.
2) NO CHEATING. Do not start UNTIL I SAY SO. And it MUST be caught.
3) You must show a picture of it and Its nature. Bonus if it's Timid/Modest! (Please provide any kind of picture of it, by Phone, DSi/3DS, Camera etc.)