How do you catch Arceus without a Nintendo event or a cheat?

Unfortuanatley, the Pokemon company has made the games so that you can only get Arceus with an event, although Hacking has changed that. If you still want Arceus, the only option is to get a certain event, getting the "Heaven's Pipe" and going back to the Spear Pillar in Mt. Corronet. Go to the symbol between the pillars and using the Heaven's Pipe. Some stairs will appear. Climb up them, and Arceus will appear.

Disclaimer: I have not tried this. I have only placed my faith in Serebii.net, which many have found true to their Word. All credit for this answer goes to Serebii.net.