Instructions
Stock up on Reviver Seeds, Stun Seeds, Sleep Seeds and Vile Seeds before you start the final fight.
Fill your battle party with Fighting- and Ice-type Pokemon, all of which should be at least level 48. Primal Dialga is particularly weak to Fighting and Ice attacks.
Use a Vile Seed as soon as the fight begins to lower Primal Dialga's attack and defense rating.
Hit your enemy with a Stun or Sleep Seed to stop him from hitting you in between your attacks. Limiting the damage you take makes this fight much easier.
Blast Primal Dialga with Fighting- and Ice-type moves whenever you get the chance.
Use a Reviver Seed if one of your Pokemon faints.
Continue balancing seed usage with your best attacks to defeat Primal Dialga. This fight will likely require several attempts, as one unlucky critical strike can KO even your strongest Pokemon.