A Pokemon Trainer who's really strong cannot be challenged again. that's a pity. however. there IS a way to challenge a trainer again. and your pokemon will sure raise by this method.

Before you challenge a trainer. save at the Pokemon Center. and fight as many trainers you want. but don't save!!!
after defeating the number of trainers.. go visit the Pokemon Centre and put the raised pokemon of your party into your pc. and change the box.when it says "SAVING... DON'T TURN OFF THE POWER."after this, turn it off. and turn it on.. if the trick has worked out. you will begin at the point you saved at the Pokemon Center.and your party is the same as before you challenged the number of trainers. but if you check the pc. your raised pokemon will waiting for ya. just release the older pokemon and withdraw the raised pokemon.and SAVE again. if you want to challenge the same trainers again. just don't forget it!

NOTE: This method will only put damage on your pokedex. Example: Evolved Pokemon, and Cathed Pokemon whill not appear cathed or seen on your pokedex while doing this trick!