i need help beating gardina in pearl. my team is prinplup lv.28 and geodude lv.9

Stop playing and think out a plan.With a notebook write Prinplup and all the moves of Prinplup and write in order what one is first or how many times to use the move to defeat her.

Try training up you're Geodude or Catch a Starly and evolve it into a Staravia. Both of these types are very effective on Grass-types.

Catch more pokemon, always use the 6 slots. ALWAYS. level up Geodude (not sure why you have one, there not great). use potions, berries etc. get a shinx, starly and if you can, get a fire pokemon.

Catch a Staravia

Level up, and catch a few more Pokemon like the others have said.

Oh come on..it's just Gardenia...catch something like...i don't know heracross or wurmple(make it evolve)

keep out the geodude and get a fire pokemon or grass and train it

I started with chimchar and I beat the game twice

I hope your Prinplup have a move peck/ drill peck. Don't use your Geodude! Remember, like other said you should catch Staravia.