How do you get Lugia in Pokemon in diamond?

Answer 1

If you own a copy of Pokemon xd gale of darkness and beat it and purified shadow lugia and have traded it to either firered, leafgreen, ruby, sapphire or emerald you can migrate Lugia to diamond then catch it in the pal park to receive it.

Answer 2

All you have to do is catch it in whatever gameboy game and then migrate it to whatever Pokemon game you have go in trophy garden walk around there

(need action replay ds code)

Answer 3

migrating or cheat