You need to have the Limited edition Coliseum for the gamecube and you are able to get it there
you use a game shark code. here's the website where i got the code! http://www.supercheats.com/gameboyadvance/walkthroughs/pokemonruby-walkthrough06.txt
Type in the Password: 73938790
You go to the Sky Pillar and you'll find Rayquaza at the top floor. Catch it six times by not saving the game after you caught it. After the sixth time of doing that fall through the black whole in the middle of the 4th floor. To fall through you need to have Jirache which can be found in victory road, because Jirache has a special move that nobody knows about until you you catch it. You ust have cut and waterfall hidden moves and you must have the technicle move charge beam. Once you fall through the whole a huge shining Arceus appears. It has great moves and you must, must have a masterball since you cant catch it without a masterball. I hope this helped you because it worked for me. : )