Well, the Elite Four and Champion in Pearl are usually pretty high-leveled, so I'd train a little more, just to be safe. Get all your Pokemon up to at least level 65 for good measure.
When battling Aaron, start with Infernape. When battling Bertha, open with Roserade, since all her Pokemon are weak to Grass-types. Next, when battling Flint, start with Garchomp. When battling Lucian, use Mismagius to start.
Save as many healing items as possible throughout the elite four challenge. Use them to heal your Pokemon right before challenging Cynthia. Finally, when battling Cynthia, open with Mismagius. Don't use your Garchomp until she sends out hers. Cynthia's Garchomp is level 66, so make sure you're ready for it. For Lucario and Roserade, use Infernape and Staraptor. For Gastrodon, use Roserade. For Milotic, use Raichu, use Mismagius for Spiritomb, and make sure to keep your Pokemon's health high. Do this and you should have an easier time. Also, I recommend saving before each elite four member and the champion, that way you can devise your own strategies through trial and error and soft reset if you lose.