I'm not sure if this works or not, but my mate told me that if u finish the Elete 4 7 times, then go to Celestic town and go into the big house in the middle and talk to the old Lady/Granny and she will show u a card with Palkia/Dialga on it then talk 2 her again and she will show u Palkia/Dialga and then say "I Have No Need For Him Anymore, Would You Like Him?" If u say yes and u have 5 or less pokemen in your party then u will get Palkia/Dialga

P.S If you don't have 5 or less Pokemon in your party u will have 2 do the 7 Elite 4 again!