Theres no way
You will need the Mysyery gift azure flute. Once you have it, go to the spear pillar where you found Dialga/Palkia and use it. There will be a LONG stairway and on top of it you will find Arceus waiting for you.
A Pokemon adventure on Pokemon Diamond Verison can last forever if you wanted it to.
as long as you want
Iron Thorn Z Pokemon Questions HelperAnswer: You can get Arceus by getting the Azure's Flute wich you can get with an AR Ds or with an event. Climb Mt.Coronet until you reach the Spear Pilar and then walk a few steps foward, a text box will ask you to use azures flute. Use it, a long stairway will appear, climb it. Arceus is at Level 80 Hope That Helped!
You have to get the Azure Flute from a Nintendo Event and go to Spear Pillar. There, you use the flute at the steps, and a long, shiny staircase will appear. Arceus will be at the top.
The Strongest pokemon... Well there isnt really the STRONGEST. Cause there are Cheats, but then it depends how much HP ups, Carbos's, ect. you give a certain pokemon. But actually, with the Azure Flute event, you could get the GOD POKEMON, Arceus. To get that, go to Spear Pillar, (WITH THE AZURE FLUTE) then play the azure flute, 1 step forward from the cave entrance. Then after a LONG time of playing the flute, a staircase would appear. Walk up it and then you would find Arceus... I recommend Saving before battling/capturing him.
with a cheat it depends if it is the one that gives you a flute then go to spear pillar and right inside the door play it and a realy long stairwell will apper go up the stairs and battle arceus. but if you have a different one there is one where the mail man will give him to you or there is one where he roams like mespirit and cresilia
as long as you haven't beat it yet and you have the azure flute, you can.
To catch every Pokemon, you need action replay for the few last diamond legendaries. You will need Pokemon fire red/leaf green and emerald. Migrate them to Pokemon diamond. In Pokemon diamond, fill up your pokedex by catching Pokemon normally or with poke radar, or use gts. It will take a long time to catch all Pokemon
the really long grass in resort area no pokeraider
You'll need to trade for it, because the event for Arceus has passed for a long time now.
no just as long as your underground