To defeat Primal Dialga/Dialga, you should be at least level 45. Make sure to bring four to six Reviver Seeds because it knows Roar Of Time, which causes a lot of damage. Whenever you can, throw a Sleep Seed at him to take a bit of health away. He has around500 HP. Also, be sure to change your partner's moves. If done correctly, you will beat Dialga and finish the game. On the second visit, after the credits, go back and defeat him again so that you can recruit him; it will help with Bosses later on.