Things You'll Need
Instructions
Play "Pokemon: Emerald Version" until you defeat the Elite Four and acquire the National Pokedex from Prof. Oak. This will allow you to collect Pokemon from previous versions of the game.
Catch or find any Pokemon you are willing to trade to your "FireRed Version" or "LeafGreen Version" game.
Go to the nearest Poke center and save your game. You can turn the console off now and remove the cartridge.
Play the "FireRed Version" or "LeafGreen Version" of the game until you acquire the National Dex for this game. This comes after collecting 60 types of Pokemon and defeating the Elite Four.
Travel to Cerulean Cave with a Pokemon who knows the move "Surf." Descend into the cave to find Mewtwo at the bottom, level B2. Save before you begin the battle. Most players use the Master Ball to capture him, although you can use other Poke Balls if you weaken him first with attacks and status effects.
Go to the nearest Poke center and save this game.
Insert the "FireRed Version" or "LeafGreen Version" game cartridge into one Game Boy Advance console and the "Emerald Version" into the other. Connect the two consoles with the link cable. If you have turned either game off, switch it on.
Go to the Trade floor of the Pokemon center on both games and initiate a trade. Follow the onscreen prompts to trade Mewtwo to your copy of "Emerald Version" in exchange for the Pokemon you caught earlier.