Things You'll Need
Instructions
Insert the Action Replay cartridge into the top of your Nintendo DS console and turn on the console.
Select the player profile in which you want to catch a "Dark Lugia."
Enter a "very high encounter rate" code, such as 94000130 FFEB0000 or 62110DC0 00000000, while holding the select and right buttons on the D-pad. You now have greatly increased the probability of finding certain Pokemon.
Enter a "rebattle Pokemon" code, such as 94000130 FFF30000, but alter the final three digits by entering Dark Lugia's Pokedex identification number of 0F9. Your final code should be 94000130 FFF300F9. This will ensure Dark Lugia appears in your next encounter instead of a random Pokemon.
Walk around in a patch of tall grass until Dark Lugia appears. With the increased encounter rate it shouldn't take more than a few steps.
Use Pokeballs to attempt to catch Dark Lugia just like any other Pokemon if you're feeling ambitious. Otherwise enter a "100% catch rate" code, such as 92247612 00002801, to ensure that even a standard Pokeball will catch Dark Lugia on the first try.