Shining Pokémon Cheat:
Step 1. Put a flying Pokémon (must have FLY) at the 1st position and put the Red Gyarados at the 6th position and put the Pokémon you wished to change in the 2nd position. 

Step 2. Put all the Pokémon back into the PC except the 1st one. 

Step 3. Take the 2nd one out, the one you wished to be changed. 

Step 4. Enter the Bug-Catching Contest, exit it as soon as you entered it; when the clerk ask if you want to quit said no. 

Step 5. Use the fly Pokémon to fly out the contest, the 2nd Pokémon should be in your inventory, store it into the PC, then go back to the Contest and said "YES" to finish the contest, now check your status screen, you should have that 2nd Pokémon you wished to change in your possession, with a different color, and has Gyarados' name. But when you entered the battle, it will still be the same Red Gyarados but don't worry, have a Rare Candy and use it on the Pokémon, and make it evolve, now when you go to the battle, it will be that evolved Pokémon with the shining status. 

NOTE: THIS ONLY APPLIES TO THE POKÉMON THAT CAN EVOLVE, POKÉMON THAT CAN'T EVOLVE WILL "ONLY" HAVE THEIR APPEARANCES ALTERED AT THE STATUS SCREEN BUT NOT THE BATTLE. AND IT WILL ENTERED A LOOP STAGE. 


Item Duplication:
Step 1. [Equip] a pokemon [with] your favorite item

Step 2. Put a flying Pokémon (must have FLY) at the 1st position and put the Pokemon [equipped with] your favorite item at the 6th position. 

Step 3. Put all the Pokémon back into the PC except the 1st one. 

Step 4. Take the 2nd one out. 

Step 5. Enter the Bug-Catching Contest, exit it as soon as you entered it; when the clerk ask if you want to quit say no.

Step 6. Use the fly Pokémon to fly out the contest, the 2nd Pokémon should be in your inventory, store it into the PC, then go back to the Contest and said "YES" to finish the contest, now check your status screen, you should have that 2nd Pokémon in your possession with your favorite item. Take the item from the Pokemon then throw [the Pokémon] away. 

Repeat 1-6 to get many items!!