Things You'll Need
Instructions
Without GameShark
Go to the Spear Pillar and use your mach bike to make it through the crumbling sections of floor until you reach Rayquaza.
Save your game before approaching Rayquaza. Approach Rayquaza and press ̶0;A̶1; to initiate combat.
Discern whether or not Rayquaza is shiny (a shiny Rayquaza will be black while a standard Rayquaza will be green). Restart the game and load from your save point if it isn̵7;t shiny. Repeat this process until you encounter a shiny Rayquaza, and then capture it (note that this could take well in excess of 8,000 attempts).
With GameShark
Connect the GameShark to your Game Boy Advance and insert the ̶0;Pokémon Ruby Version̶1; cartridge.
Select ̶0;Code Entry̶1; and enter the master code for ̶0;Pokémon Ruby Version.̶1; It is: 928817F0298A 50F818720DD7 E005BA4B98A5.
Enter the shiny wild Pokémon code, which is: 81FE44749C55 A0FA44FCCC7F 11B25A7BD975 56FC0D142272 D9A856E44D4C.
Enter the Pokémon modifier code for Rayquaza, which is: 05C2A2F36845. Note that this will make Rayquaza the only wild Pokémon you can encounter while the cheat is activated.
Start the game and enter a grassy area to encounter a shiny Rayquaza. Capture it as you would any other Pokémon.