Instructions
Pal Park
Make your way to Pal Park on Route 228.
Insert a cartridge of any Game Boy Advance Pokemon game into the GBA slot of the Nintendo DS. Make sure that the cartridge contains a game file in which Shroomish has been caught.
Talk to the Pal Park attendant to begin the mini-game. Select six Pokemon to transfer from the Game Boy Advance game to "Pokemon Diamond."
Catch all six Pokemon in the mini-game. You now have Shroomish in your PC.
Trading
Go to any Pokemon Center and have your partner do the same. Make sure your partner has a copy of "Pokemon Pearl," "Pokemon Diamond" or "Pokemon Platinum" that contains Shroomish.
Press "A" to speak to the trade attendant and initiate the trading procedure.
Select the Pokemon you wish to trade. Have your partner select Shroomish.
Confirm the trade by selecting "Confirm." Shroomish will now be in your party.
Safari Zone
Travel to the Safari Zone. Make sure you already have the National Dex.
Walk around the Great Marsh section of the Safari Zone. You have a 10 percent chance of encountering Shroomish.
Use a Safari ball to capture Shroomish. If it escapes, keep roaming around the Great Marsh area until you encounter and capture one. Once you have captured it, it will be sent to your PC.