(Pokemon Platinum) Can I beat Cynthia with this team?

You need more training. Have you seen her team? She has (if i reember correctly) a lucario, Roserade, Milotic, Garchomp, Gastrodon and a Spirittomb, all of which in lv. 65-67. You need to train a little more. Your giratina may take out her Garchomp, but if it's not trained more, Garchomp will have more speed and take it down as it does know Dragon rush. And judging from your team, looks like Milotic will at least take more than half down, since it knows water and ice type moves, so i would suggest replacing one of your fire types with a Luxray, since you do have two. hope i helped