Pokémon Diamond and Pearl/Hall of Origin

The Hall of Origin isn't really a "Hall" it's more like a tiny glass platform above the Spear Pillar with Arceus on it.

The only way to get there is to cheat the game to give you the Azure Flute in Mystergy Gift. NOTE: It won't work if you just cheat the flute into your bag, you MUST receive it in a mystery gift. This can be done with either an Action Replay code or using Pokesav.

After receiving it, go all the way up Mt. Coronet and into Spear Pillar. Once in, the game will ask you if you want to play the flute. Well of course you do! Play it and a stair case will appear. THIS IS WHERE YOU SAVE. Once you get to the top of the stairs you will challenge Arceus automatically.

#493 Arceus

Arceus is at level 80, the highest level any wild Pokémon has ever been to date. He has amazing stats (also the highest base stats out of any Pokémon to date, at 120 each). He knows the moves Refresh, Recover, Hyper Beam, and Future Sight. Recover and Refresh make him very hard to catch, because he can Refresh away any status conditions and Recover any damage. If you have a Palkia or other pokemon that knows heal block, it would be possible to prevent Arceus from using Recover. (Palkia learns Heal Block at level 50.)Still, you're going to have to get lucky to catch him.