Instructions
Defeat the Elite Four after collecting all eight Gym Badges from the Pokemon Gyms scattered around the Sinnoh Region.
Return to Mt. Coronet and enter the room on the fourth floor by using the Pokemon moves "Surf" and "Waterfall." Pokemon that know the moves "Strength" and "Rock Climb" will also be needed to move boulders from the path and to travel up cliff faces.
Pick up the Adamant Orb off of the floor inside of the fourth floor of Mt. Coronet. The small section of dry ground propping the Orb up is reached by using "Waterfall" from the third level.
Climb to the summit of Mt. Coronet and enter the Spear Pillar temple with the Adamant Orb to encounter Dialga. Save the game before walking up to Dialga because it is a difficult Pokemon to capture.
Use Fighting or Ground-type attacks to heavily damage Dialga until its health bar turns red. Use Ultra Balls, Heavy Balls and Timer Balls on Dialga when its health is low. Ultra Balls are a generic heavy-duty Poke Ball, while Heavy Balls have a better chance of capturing weighty monsters and Timer Balls work best after many turns have passed.