Instructions
Prepare for the battle. Obtain Sleep Seeds, Reviver Seeds and Stun Seeds. Bring dragon-, fighting- or ice-based Pokemon that match up against Dialga's dragon- and steel-based characteristics.
Throw Sleep Seeds at Dialga as soon as the battle commences. Keep attacking until he wakes. Use another Sleep Seed or a Stun Seed to keep him immobilized. Repeat this process.
Respond to Dialga's attacks by healing. His move, the Roar of Time, does huge damage, so heal yourself as soon as it strikes. Use Reviver Seeds if anybody in your party is knocked unconscious. Heal yourself even if you have half of your health left.
Attack Dialga when you're not healing. Alternate between throwing Stun Seeds and Sleep Seeds and attacking. Counteract Dialga's Intimidator IQ skill by using ranged attacks. Stay on the offensive until Dialga is defeated.