Dialga

In order to get dialga, after you beat the elite four, and you have the national pokedex, go to the old grandma in Celestic town and talk to her. She'll tell you stuff about them. then go up to Mt. Coronet and use rock climb to the fourth or fifth level (i forgot which. sorry!!! it's one of them.), use waterfall and grab the adament orb and the lustrous orb. Go to where you got sucked into the reverse world and you'll see two floating lights. One's blue (Dialga) and one's purple (Palkia). click on the blue one and choose yes and you will battle him.

If you've played Pokemon Diamond, you probably came prepared for Dialga; he's a legendary. Though anywhere on the Internet will say "he's one of the easiest legendaries to catch", he's a legendary nevertheless, so you come prepared. And it pays off, because he's still not easy to catch. But on platinum, it's different. If you want to preserve yourself the "Crud! I hate this guy!", you don't need to waste 50,000 dollars. On platinum, he only took me two duskballs and an ultraball to catch. Also, don't do the same mistake I did with Palkia. I assume that Palkia is just as easy to catch, so don't use your masterball on him. I still hate myself for that mistake.

Now, it might be scary that he's level 70, while your only level 60, but it doesn't matter. As long as you can sacrifice 2 or 3 Pokemon, you can catch him. It's that easy!