How to Catch Arceus on Pokemon Pearl

Go into a Pokemart and look for the man dressed in green. Walk over to him and talk to him and receive the Azure Flute. Leave the Pokemart and check your map.

Head towards Mount Cornet and make your way up. Use repel balls to avoid fighting any Pokemon you meet along the way, and use Surf and Scale to cross water and climb small walls. At the top of Mount Cornet is the Spear Pillar; this is the area where you fought Dialga earlier in the game.

Step on to the stairwell leading up to the Spear Pillar. A message will inform you that your Azure Flute has begun making noise. Select "Yes" to blow into the flute. Flute music will play briefly, then an extension will appear on the staircase which leads above the level of the Spear Pillar. Ascend this staircase. At the top you will find Arceus.

Call a Fighting-type Pokemon from the Pokemon you have with you. Arceus is weak towards Fighting-type moves. Examples of these Pokemon include Hitmonchan and Machamp. Arceus will repeatedly heal itself, so use a move like Revive Block to prevent Arceus from doing so. Then, use Fighting-type moves to inflict the most damage. Continue to do so until Arceus is at a very low level of health, then try using paralysis moves like Bind.

Throw a Pokeball at the weakened Arceus to capture him. It is advisable to use an ultra ball or something similarly high because, even in its weakened state, Arceus can be resistant to capture.