b r o t h e r n e r o
I think I may have an idea for a Colosseum/XD challenge. How about you can only catch all the Shadow Pokemon with a Pokeball? You can still catch a pokemon with a Great Ball or Ultra Ball, but if you do, you must use that pokemon on your team. (meaning you can only use a ball other than a regular Pokeball 6 times, for each 6 pokemon on your team) Every other pokemon besides those 6 must be caught with a Pokeball. You can't skip a pokemon either; you must catch all of the Shadow Pokemon in the game.
Hopefully that was semi-understandable.