Things You'll Need
Instructions
Insert a copy of "Pokemon Ruby," "Sapphire" or "Emerald" into your Nintendo DS's GBA slot and catch a Tropius. Tropius can be found on Route 119.
Save your game after catching Tropius, then turn off your Nintendo DS.
Insert your copy of "Pokemon Diamond" into the DS cartridge slot and turn on the system. Launch "Pokemon Diamond."
Scroll down and choose the option to transfer Pokemon instead of selecting "Continue." Choose Tropius and five other Pokemon to transfer over. You must transfer six Pokemon at a time, so consider sending over other Pokemon that would be otherwise unavailable in "Pokemon Diamond."
Select "Continue" and resume playing "Pokemon Diamond" after you've chosen the Pokemon you wish to transfer.
Head to the Pal Park on Route 221 after you've defeated the Elite Four and gotten the National Dex.
Explore the Pal Park until you encounter Tropius somewhere in the tall grass, then use a Park Ball to catch it. You receive exactly six Park Balls when you enter the park.