Instructions
Start your adventure through Pokemon Diamond, catching Pokemon and making them stronger. The Pokemon in your party will need to be at level 45 or higher to take on Dialga.
Travel along route 207 and enter Mt. Cornet.
Proceed through Mt. Cornet and make your way to the top. Your Pokemon must posses Surf, Rock, Climb, Smash and Strength to reach the top.
Reach the Spear Pillar at the top of Mt. Cornet and defeat the trainers that challenge you.
Approach Dialga and it will challenge you to a battle.
Send out a quick Pokemon to get in the first attack. Dialga is a large, slow Pokemon whose attacks are strong, therefore it is advantageous to attack first.
Attack Dialga with Ground and Fighting moves. Avoid sending out Grass or Poison Pokemon because they will have almost no effect. Dialga will have 100 HP, so remain persistent even if your first Pokemon faints.