Instructions
Stock up on healing items at the Eterna City Poke Mart. Gardenia's Roserade knows moves that can poison and paralyze your Prinplup, so buy Antidotes, Paralyze Heals and Super Potions to use during the battle.
Equip your Prinplup with an item. A Cheri Berry or a Pecha Berry, both of which can be found along Route 205, are good options, as they heal any paralysis and poison your Prinplup experiences in battle.
Heal your party at the Pokemon Center immediately before you challenge Gardenia to a battle.
Use the Peck move in battle. The Peck move only has a base power of 35, but because it is a flying-type Pokemon move, it does double damage against Gardenia's team of grass-type Pokemon. Piplup learns Peck naturally at level 15 before evolving into Prinplup.