Do you need phione to get the national dex?

no, you need to see all the Pokemon native to the shinnoh region including both palkia and dialga, and the trio. and you can get phione you just breed manphy if you go to the trophy garden on route 212 go to mr. backlot's office on the table behind him there is a book it has manphys details in it then go to jubilife city and go to the global terminal and get manphy and breed it with a ditto if you don't have ditto get it from emerald or ruby or sapphire and there you go hope this helps