The second gym you have access to in Pokémon Shining Diamond and Shining Pearl resides in Eternia City. Gardenia is the leader of the Grass-type gym and is protecting the Forest Badge. Gardenia will not challenge you until you have completed the gym puzzle. Each of the trainers in the gym is hidden in a specific order. Find and defeat them to get clues about the location of the others. After you have taken out all the trainers, you are free to take on Gardenia. She awards the Forest Badge, Stickers, and Grass Knot, along with the ability to use Slash in the Underworld via the Pokétch.
Gardenia has two Pokémon that you can mostly run through. Cherubi is its protagonist and has a useless skill without sunlight. You can't critically hit Turtwig due to Shell Armor, but as long as you're not eating Razor Leaf, you should be able to use any Fire, Flying, or Bug-type moves against them.
If you are under-leveled (which is difficult to do) Roserade is a problem as it could outperform most of your team. Bring Fire, Flying, or Psychic attacks and it will fall. Stun Spore could make things scary, so we recommend that you bring Paralyze Heals. Technician only powers up Poison Sting, so ignore him. Just hit Roserade with physical attacks to avoid reverse.
Gardenia won't give you a Leaf A sticker without a fight. You have a Sun team and you know how to use it. Jumpluff will keep things going, trying to use Sunny Day to power up his teammates. A good Pokémon with Taunt can ruin those plans. Gardenia's Breloom is a Sub + Puncher with Toxic Orb to heal. A Pokémon with Switcheroo or Trick could start with any less useful item. We suggest replacing the item with a chosen headband / scarf after seeing a non-damaging move as a Substitute.
Active sunlight means that all sun rays fire instantly and even without sunlight, Roserade can fire instantly with Power Herb's ability to cut reload times in half. Roserade's game plan hasn't changed since your first fight. Make him pass out before she can sweep. Torterra is four times weak to Ice-type attacks, so prepare an Ice Beam or something similar to avoid her devastating attacks.