How can you get arceus

complicated. you could go to a Nintendo event, or use an action replay , which costs 20 bucks. ( Nintendo events are real. you actually have to go somewhere in real life.)
That and you need the azure flute which idk how to get sorry and if you get it from the AR or Nintendo event go to where you got palkia or dialga and play it. Stairs will soon apear going up even higher and arceus will be there. be careful it is lvl 80 and is tough to get if you don't have an AR becuause it repels masterballs sometimes.  Arceus, from what I've seen, is meant to be the god of all Pokemon and is the highest level wild Pokemon possible to face without an AR or hack. Arceus is obtainable from only a item received from an event called the Azure Flute. Go to the summit of Mt. Coronet (aka Spear Pillar). Use the Azure Flute to play a tune. Transparent stairs will appear. Climb them and you will find Arceus. Arceus is a powerful Pokemon at Lvl. 80 and the moves it deals are absolutely deadly, which could spell doom for your Pokemon if they aren't the same level or higher. Arceus can use a move called Judgment and that's it's secret weapon. Try unleashing Fighting-type moves on it and then use a lower level based Pokemon to weaken it. Sleep and Freeze conditions will help IMMENSLY. Once it's low enough, throw a Master Ball/Ultra Ball/Dusk Ball (Since you are technically still in the cave, it counts as night.) If you got lucky, you now own the most powerful Pokemon in the world!