First off, I would get your Pokemon to level 58 or higher. Cynthia's Garchomp is level 62, so you might want to be near that range. After your Pokemon are at a high enough level, challenge the Elite 4. When you get to Cynthia and she leads will Spiritomb, hit it with your best Pokemon. You have to do this because it has no weaknesses in gen 4. I suggest hitting it with an Earthquake from your own Garchomp. Then for her Garchomp, teach Floatzel Ice Beam. The Floatzel should out speed it and knock it out in 1 or 2 turns. Then with her other Pokemon just get a good type matchup and you will be fine.
So the most important thing is just get your Pokemon to a higher level.
Hope I helped!