okay here's how to beat the elite-4 on Pokemon platinum{sorry i don't now how to do it on diamond}first you need a rock tipe for Aaron a ground tipe a fire tipe to defeat
Aaron.Next to defeat bertha you need a grass type a water type.to defeat flint you need a rock type and a water type.To defeat Lucian you need a bug type a fire type.
to defeat cythnia you need a ice type a fire type and a electric type good luck!
oh yeah heal you Pokemon before you step in Cythnias room because as soon as
you step in she will battle you.oh and if you have platinum version get the guide for it's
at game.
You eventually have to beat her. She is the champion to the elite 4. So after you beat them you beat her, become a champion, and your Pokemon go into the hall of fame. So choose your favorite Pokemon!
Beat all the elite four and there champion
beat the elite four and shes the champion
Beat the Champion, talk to the boatman in Snowpoint City.
You have to beat the guy who wants to to use Pokemon for no good.... then you can get the Pokemon..... if you beat it then you'll have to wait until you beat the champion.
u must first complete your pokedex and beat the champion
You have to get all 7 Gym Badges, beat the Elite 4, and defeat the Champion, Cynthia.
you do not see her any more you only will see her at celestic town after you beat crasher wake the 4th gym leader
The Pokemon Champion is Cynthia
The Pokemon league is where after you win 8 badges, you get to battle the elite 4 (four tough trainers) and the champion of the region (in diamond it's Cynthia until you beat her).
It never ends, but to end the basic story you have to beat Aaron, Bertha, Flint, Lucian and Cynthia (Elite four and Champion).-
Cynthia