The Hall of Origin isn't really a "Hall"- it's more like a tiny glass platform far above the Spear Pillar with Arceus standing guard over Sinnoh from atop it.
The only way to get there is to get the event-item Azure Flute. If you can't get the item then the only other way to get Arceus is through trading for it.
After receiving the flute, go all the way up Mt. Coronet and into Spear Pillar. The very moment you arrive at Spear Pillar, the Azure Flute will start making noises, and you'll be asked you if you want to play the flute. Well... of course you do! Play it and a magical stair case will appear. If you don't have a Master Ball in tow or wish to avoid using one, THIS IS WHERE YOU SAVE. This is going to be an outrageously challenging battle, and should you white out, you'll be banging your head against the wall for not saving. Once you get to the top of the stairs, you'll come face to face with Arceus, the Alpha Pokémon. It will bellow a territorial roar, once as you appear before it, and then again as you approach it, this time more agressively, prepared to fight you! There's no backing out now- get ready to face down the strongest Pokémon of them all!
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 Palkia or other Pokémon 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. Hyper Beam, coupled with STAB bonus from Arceus's Normal-type, will do beastly damage to any Pokémon that don't come close to its stats or level. Sending out a wall that's good at inflicting status conditions is a pretty good idea.
Your best bet against this overwhelmingly mighty creature is, without a doubt, Sableye or Spiritomb or just simply throwing your masterball as this is the hardest Pokémon to catch in the game. Sableye and Spiritomb are unaffected by both Normal and Psychic attacks, rendering Arceus incapable of harming one unless it runs out of moves and becomes forced to use Struggle. In addition, some Ghost Pokémon can know Spite, which can whittle away the PP of Recover and Refresh until Arceus can't use these moves, making its capture that much easier. However, it's a double-edged sword: Normal-types are unaffected by Ghost-types, which means you'll have to bust out a Ghost Pokémon with at least one move of a different type to harm Arceus. The best choice would be a Sableye or Spiritomb that learned a Fighting-type move via Technical Machine, because it can expolit Arceus's weakness against Fighting-type moves while remaining untouchable. However, Sableye can't learn Spite, which means you need to swap between it and a Ghost Pokémon that knows the move to wear down Arceus properly. Or you can bust out a Lv.20 Shedinja and slowly troll this thing to the verge of defeat, then give it a status problem and start chucking those Ultra Balls.
If you can wipe out the PP of Refresh and Recover, try to freeze Arceus. Poisoning or burning it will mean Arceus will eventually faint, paralysis is dependent on luck as to whether it impairs Arceus or not each turn, and sleep will always will wear off. A frozen Pokémon has to thaw out and miss a turn, and the ailment won't go away unless the Pokémon successfully thaws- a 20% chance per turn.
If you employ all the strategies listed here, Arceus will be at its weakest and primed for capture. If you succeed, buy yourself a 24-karat gold trophy- you have the ultimate Pokémon among your ranks now!
Arceus may not look like much as a plain old Normal-Type, but what truly makes it unique is this Pokémon's Special Ability- Multitype. You've most likely picked up a few Plates on your journeys through Sinnoh. At first glance, they might appear to be nothing more than amplifiers for a Pokémon of a specific type, but there's a much greater power they hold. These Plates were created by none other than Arceus himself! With them at his disposal, he can harness the different abilities of the seventeen types. All you have to do is give Arceus a Plate to hold, and he will change into the type the Plate was made for. This means that depending on what Plates you equip him with, Arceus can recieve STAB bonus from virtually any move!! On top of that, he can learn almost any TM and all HM moves, so you can fine-tune his moveset for any opponents you want to face. However, you can't equip all the Plates at once, so don't go thinking you've got an untouchable god at your disposal. Arceus also adopts the weaknesses of each type whenever he wears the Plate. This means that while you can wipe out the weaknesses he has as one type, he'll gain other weaknesses in their places. Arceus would be too overpowered and the game would lose all sense of difficulty if this didn't happen.
Although Arceus is unquestionably powerful, it can learn one move that makes it the supreme champion: Judgement. Arceus will launch a punishing strike that changes to match his type if a plate is attached, which will always result in a STAB bonus. However, this move is only learned at his zenith- LEVEL 100. That's right- you have to raise him to his maximum level to get this move in his learnset- no excuses. But when he does... whoo boy, watch out! Nobody will be able to call you a pushover in battle ever again!