Instructions
Fly to Oreburgh City and walk to the nearby entrance to Mount Cornet. Travel along the trail on Mount Cornet. At one point you will need to use the Surf ability to cross the water, then later the Rock Climb ability to scale a wall.
Battle with Team Galactic Grunts and Admins along the path of Mount Cornet. Select your toughest Pokemon and use your strongest special attacks to make the battles quick and easy. Continue along the path until you reach the Spear Pillar, where you will be confronted by Team Galactic's Boss.
Prepare for a long battle before walking up to the Team Galactic Boss at Spear Pillar. You will first have to battle the Team Galactic Boss, then immediately after the battle you will have an encounter with Palkia. You must survive both battles, so heal all of your Pokemon ahead of time and put your highest leveled Pokemon on the team. You do not have to catch Palkia to obtain Dialga in your Pokedex, but you must at least defeat it in battle.
Fly to Celestic Town and enter the biggest house in town. Talk to the old woman inside and she will show you a picture of Dialga, which gives you the Pokemon's entry in your Pokedex.