You need to win 200 battles in a row...then go to Hearthome city, than to talk to Mr. Good, he'll then congradulate you and give you the Azure Flute.
With the Azutre Flute go to Spear Pillar(Mt.Cornet to the top)and when you arrive to Spear Pillar use the Azure stairs will appear go up your going to see Arceus on floating glass,save and battle him.
Good Luck