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. The 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 and completed the Sinnoh Pokedex.
Non-Hack Instructions: Once you have entered the cave on Victory Road you will go down a flight of stairs head north and descend a second flight of stairs and enter the next door at the bottom. Upon entering this room you will notice a thick fog and need to use Defog. Once you have cleared the room a girl will be standing there with her back turned to you. Once you start to move into the cave she will ask to join you and keep your Pokémon healthy to get the end of the cave, she will then join you. Once you get through several trainer double battles you will go through the exit cave that will lead you to a field of grass and flowers; Note: that Mindy the trainer who joins you is pretty useless she has an Arcanine which only ever uses the move Helping Hand which does absolutely nothing to help you in the battle. Once you reach the exit Mindy will depart from you and tell you she is interested in seeing the giant stone at the end of the next section and just look at it, what she really does is uncertain however, you now depart and can go through the exit also. (This part of the level will be added soon, I am currently working on it myself)
Note: This part requires you to have an Action Replay Cheat. 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.