Things You'll Need
Instructions
Pal Park
Catch Lugia. It can be transferred to Pokemon Diamond from Emerald, FireRed or LeafGreen. There is only one in each game and it is located at Naval Rock. It is a good idea to save the game before you attempt to catch it so you can try again if something goes wrong.
Save the game onto a Game Boy Advance flash cartridge. Simply insert it into the card slot and select the save to card option from the menu. This means Lugia can be transferred over to a Diamond game.
Obtain the National Dex. This can only be achieved when you have seen all the Pokémon available in the game. It involves battling every trainer and seeing all one off Pokémon in the wild. Go to Professor Oak to receive it once you have done this.
Go into the Pal Park. This is located on Route 221 and you can simply walk in after receiving the National Dex.
Transfer Lugia over. When you're in the Pal Park, insert the cartridge into the card slot on your Nintendo DS. Go back to the title screen and instead of clicking continue, scroll down until you see an option to transfer over Pokémon. You can then select up to six at a time.
Trade
Find someone with Lugia. If you have Pokémon playing friends, ask around to see if any are willing to trade. If not, there are various online communities, such as bulbagarden.net.
Obtain a friend code and pal pad. In the game, go downstairs in any Pokemon center and speak to a woman at the desk. She will give you a friend pad. Log onto the Wi-Fi from the same room to get the code.
Exchange friend codes. Add the friend code of the person you're trading with by going into the pal pad and typing the number.
Complete the trade. Making sure you both have the correct Pokémon in your party, you must both connect to Wi-Fi in the downstairs area of the Pokémon Center. You will then be able to see which of your friends are online, before simply selecting your friend, offering a trade and choosing the appropriate Pokémon.