Flower Paradise is home to the Legendary Pokémon Shaymin. As of now, the only way to get Shaymin and access to Flower Paradise is to either cheat with an action replay or trade with someone who has done so. He Flower Paradise is only accessible (with walk through walls) through Route 224, the route in Victory Road that opens up once you have defeated the Elite Four.
At the end of Route 224 there is a small gap between the water and another strip of land. Using walk through walls, walk across the water until you are on land a few steps away. Then hop on your bike and cruise through about two minutes of flowers. At the end on a stump of ground is the Legendary Pokémon Shaymin.
Legendary: Shaymin
As with all Legenday Pokémon, save before fighting. Shaymin is a Grass-type at Lv. 30, and knows Defense Curl, Magical Leaf, Leech Seed, and Synthesis. An idea would be to bring five Pokémon of low level (preferably other Grass-types) for lowering health and a high-level Fire-type for defense in throwing the Poké Balls.
Use Heal Block or put it to sleep. Whittle it's HP down little by little. When it's in the red zone, switch to the Fire-type and Start tossing Ultra Balls, or Timer Balls in a long fight. Using False Swipe is recommended. If you kill it, you will never get it again unless you trade for it. Shaymin is very hard to catch, so if you find it, be sure to save before your battle just in case you kill it.