Revisiting Spear Pillar for Dialga and Palkia
After you have become the Champion of the Sinnoh League, you will have the opportunity to encounter and capture the box art legendary Pokémon, Dialga and Palkia, if you missed them during the main story or wish to capture them again. To do this, you will need to return to the Spear Pillar, located at the northernmost point of Mt. Coronet. This is the same location where you faced Team Galactic and the legendary Pokémon.
Upon arriving at the Spear Pillar, you will find that the area has changed. The remnants of Team Galactic's operations are still present, but the legendary Pokémon themselves will reappear. Be prepared for a challenging battle, as Dialga and Palkia are extremely powerful. It is highly recommended to save your game before engaging them, as you will want to ensure you capture them.
Dialga, the Temporal Pokémon, is a Steel/Dragon-type and is weak to Fighting and Ground-type moves. Palkia, the Spatial Pokémon, is a Water/Dragon-type and is weak to Dragon and Fairy-type moves. Having a well-leveled team with strong offensive Pokémon is essential. Consider using status-inflicting moves like Sleep or Paralysis to make the capture process easier. Ultra Balls are generally the most effective Poké Balls for capturing legendary Pokémon, but Dusk Balls can also be useful if you are battling at night or in a cave.