Instructions
Level up your Pokemon team to level 70. While it is possible to fight Shadow Lugia under this level, you will not win.
Walk around in grassy areas of the game world to be attacked by grass-type Pokemon. Both Lugia and Shadow Lugia live in the grass, but as Shadow Lugia is the rarest Pokemon in the franchise, it will take hours of wandering before it appears.
Capture Shadow Lugia as you would any other Pokemon. Weaken it by battling it with your team until you can capture it with a Pokeball. If you have a Master Ball, which captures Pokemon instantly, use it, because Shadow Lugia is incredibly powerful.