This guide will show you how to beat Gardenia in Pokémon D/P. Before I begin, let’s have a look at her Pokémon:
Her Pokémon are all Grass Pokémon. Grass Pokémon are weak against Fire, Flying, Bug, Poison and Ice. Roserade is Grass/Poison, so her weakness are Fire, flying, psychic and Ice.
As your Fire Pokémon, you have Ponyta and Monferno. For your Flying Pokémon, you have Staravia and Zubat. If you have a bug Pokémon, it should be Beautifly, Dustox or Kricketune. Your psychic Pokémon, if you have one, would be Abra. But I’ll focus on the Flying and Fire Pokémon, since those are the Pokémon that are the easiest to get.
Cherrim is an easy one. IT can’t do you much, and have low defenses. You should be able to beat him easy, even without super effective moves.
Turtwig is harder, he has Razor Leaf. His defense is quite high, so I suggest to use special attacks, or supper effective attacks. I recommend to use Fire or Flying Pokémon (with Wing Attack) against Turtwig.
Roserade is hard. Roserade has good moves like Grass Knot and a very high Special Attack. It can also paralyze you with Stun Spore. Use quick Pokémon with Super Effective move like Staravia and Monferno to beat him.
Grass resist Electric, Ground, Grass and Water. So don’t use those move against Gardenia’s Pokémon.