Instructions
Purchase several Dusk Balls and Ultra Balls from any shop. Also ensure that you have sufficient healing supplies for your Pokemon.
Travel to Mt. Coronet, located north of Pastoria City.
Travel through Mt. Coronet by following the path, until you reach the Spear Pillar at the mountain's peak.
Battle the two Team Galactic Grunts that stand in your way. They must be defeated before Dialga can be confronted.
Save your game and fully heal your Pokemon. Dialga is a few steps to the north of where the Team Galactic Grunts were encountered. Saving the game will provide a failsafe in case you accidentally defeat Dialga instead of catching it.
Speak to Dialga. This will begin the fight with Dialga.
Use an attack that will Paralyze Dialga. Paralysis greatly increases the catch probability.
Reduce Dialga's HP to a minimal amount, but without reducing it to 0. When Dialga's HP is at 20% of maximum or lower, it will appear in red.
Throw a Dusk Ball at the weakened Dialga. Dusk Balls have the highest catch rate against this type of Pokemon.
Continue to throw Dusk Balls at the weakened Dialga until it is caught. Even with Paralysis and low HP, there is no guarantee that it will remain in the Dusk Ball on the first throw. Multiple attempts may be necessary.