You need 2 players (or 2 R/B/Y carts), and a link cable.
Second player gets a Pokémon with Dig.
First player raises a Pokémon with the desired special value, and this Pokémon is the only Pokémon in their party.
Second player does the "Alternative Method" of the Ditto Trick, but stops after step 5 (beating their rival)
Second player does the Glitch City trick and stops after step 4 (when they go out of the Safari Zone after saving and resetting).
Both players link up to battle
Second player defeats first player, and uses up 500 steps in the Colosseum.
First player resets while second player is walking around.
Second player goes back inside the Safari Zone after being warped there, then immediately uses Dig (the start menu should show RESET instead of SAVE), flies to the town nearest to where they flew away from the trainer and walks to where they flew away from the trainer.
Second player's start menu pops up, and it gets dismissed; a wild Pokémon should appear, whose HEX is equal to the first player's Pokémon with the desired special. This Pokémon gets caught.
Second player changes his Pokémon Box to save.
Optionally, for both players to have the Pokémon in their Pokédexes, the second player should reset, withdraw the Pokémon if necessary and trade it to the first player.