I can't beat Gardenia in Pokemon Platinum!!!?

You should try using prinplups peck against her. If you are still having trouble, you can catch a ponyta, fire type outside eterna city to the east of it. Level it up till it learns ember, and you should have no problem with the gym. Staravia's are also effective against it. She has a cherubi, a turtwig, and a roserade. Use primplups peck on the first two, and then use ember in the last one.