Instructions
Walk toward Dialga until you enter the battle.
Move your arrows until you highlight "Items" and press the "A" button. Select a Sleeper Seed, Stun Seed, Blinker Seed or Totter Seed to temporarily incapacitate Dialga. Your Pokemon will attack Dialga with the item.
Open the "Moves" menu. Select a move to attack Diagla. Diagla is a Steel- and Dragon-type Pokemon, so he is weak against electricity, fire and water. Attack with electricity, fire and water moves for the best chance at defeating Dialga.
Attack Dialga with a new Sleep Seed whenever he awakens.
Use a healing item on your Pokemon when his hit points drop into the red. Healing items include reviver seeds and Oran berries.
Continue attacking Diagla until you defeat him.