Go to the Orebeurgh Side of Mt. Coronet after you have HM8 Rock Climb which is behind a house on Route 216. Then use Surf go down,right,climb the wall and you're on the way to Spear Pillar to catch Diagla/Palkia. There will be some Galatic Grunts to stomp first though. Then at Spear Pillar, You're friend comes and helps you stomp Mars and Jupiter then you kill Cyrus and then you battle (and TRY to catch) If you used up your Master Ball, that's fine! Bring Ultra Balls or Net Balls! That's how I captured Palkia. You can also used some Quick Balls too.