If you mean Dialga...
Go to mt. Coronet, and you have to make your way through a certain place (I believe it was through the eterna city entrance). when you get far enough, you should see the team Galactic leader, Cyrus, awakening Dialga (or Palkia in Pearl). I have heard many sources tell us not to use the master ball. Well, it is possible to use a dusk ball or an ultra ball, just stock lots, and make sure that you save before spear pillar. Dialga isn't hard to catch, but he's really hard to avoid killing. So stock a lot of Pokemon who can inflict status conditions
Well first you need to free the fairy Pokemon trio. Then go to mount coronet. After that you have to fight some galantic people then catch or defeat diagle. Then go to sunnyshore city. First you see flint at gym then go and get the gym leader Volner from the lighthouse. Hope that helps.
mt.corenet top floor
To get Dialga, you have to get a friend to trade it to you. To see Dialga, go to the big house at the north end of Celestic Town and talk to the elderly woman.
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