Instructions
Enter battle with a stock of at least 10 Reviver Seeds. The more Reviver Seeds you bring to battle, the better. Expect that you or your Pokemon partner will become knocked out throughout the battle. Prioritize their revival whenever they're knocked out. Dialga's "Roar of Time" attack is especially damaging.
Attack with Bulbasaur's Ice attacks, such as Ice Beam and Blizzard. Bulbasaur's regular Water attacks will cause damage, but the Ice attacks will cause more damage.
Attack with Torchic's long-range attacks, such as "Flamethrower." Dialga isn't particular weak against Fire attacks, but using long-range attacks allows you to keep Torchic out of Dialga's immediate range. This minimizes the damage to Torchic.
Throw Sleep Seeds or X Eyes Seeds. They won't always stick on Dialga, but when they do, they will temporarily cause Dialga to stop attacking or to miss while it attacks.