Things You'll Need
Instructions
Connect your Game Boy Advance to your GameCube using a GBA connector cable.
Insert Pokemon FireRed into your GBA and Pokemon Colosseum into your GameCube. Power on both consoles.
Navigate to the Pokemon Center in Pokemon Colosseum, which is located in Phenac City. Go to the basement of the Pokemon center and activate the Trading Station.
Navigate to any Pokemon Center in Pokemon FireRed. Go to the second floor of the center and speak to the girl at the counter of the Cable Club. Confirm that you wish to connect to another player.
Select the Pokemon you wish to trade from "FireRed" to "Colosseum." This should be the Pokemon you wish to clone.
Confirm the trade on both consoles. Note that after the trade takes place, FireRed will automatically save while Pokemon Colosseum will not.
Save your Colosseum game and re-enter the trade menu with the FireRed game.
Trade the Pokemon back from Colosseum to Pokemon FireRed. Do not save on Colosseum.
Reset your GameCube and reload your save game in Colosseum. Your traded Pokemon will be back in your Colosseum party.
Enter the trade menu with FireRed and select your cloned Pokemon from Colosseum. Trade it over to FireRed. Your FireRed will now have two of the exact same Pokemon.
Repeat as many times as you want to create multiple Pokemon clones.