Well, Primal Dialga is easy if you ask me.It is a good thing to have a lot of Sleep/Parlyze seeds and If you are lucky some Reviver Seeds.Dialga is Steel/Dragon. So Ground, and Fighting type moves have the advantage.If your team can learn Dig then it's very easy to beat him.The first time I beat him, I used Chimchar and Turtwig.Chimchar knew Dig and It was pretty easy.Also if you're lucky you can avoid Roar Of Time which is a devastating attack for your team.