For 1 thing. Lugia is NOT in the seafoam islands. It is in the whirl islands. If you are in the seafoam islands, There is Articuno, not Lugia. To catch Lugia, I would reccomend getting a Freezing or Sleeping status condition move. Then it can't attack and won't run out of PP and use Struggle. Through Ultra or dusk balls while it is sleeping or frozen. When it wakes up or thaws out, put it back to sleep or freeze it again and through more Ultra and Dusk Balls. Keep using this Strategy until it is caught. The same thing if you ment Articuno, not Lugia.