How do you beat Cynthia?

In Diamond and Pearl, Cynthia's party consists of Spiritomb, Roserade, Gastrodon, Lucario, Milotic and Garchomp, in order to beat Spiritomb you will have to use your most powerful attack since it doesn't have any weaknesses however for Roserade you can use Fire-type Pokemon such as Charmander, Charizard, Flareon, Moltres, Cyndaquil, Tepig and Emboar, Gastrodon you can use Grass-types such as Bulbasaur, Chikorita, Meganium, Hoppip, Jumpluff, Snivy, Vileplume and Oddish, with Lucario you can use Fire-types, you can use Electric-type Pokemon to beat Milotic and you can use Dragon and Ice-type Pokemon in order to beat Garchomp.