How to get a lugia with multiscale?
Multiscale is Lugia's Dream World/Hidden ability, Multiscale Lugia can only be (legally) obtained through the Dream Radar, and only then when a SoulSilver game card is inserted into the 3DS. Source:
For those who have not seen, it was revealed that Ho-Oh and Lugia were released in the Dream Radar with their Dream World abilities. - Smogon
Lugia as of now is not avalaible in the dream world with its ability.Nintendo might release a future event for it.But you can get in in Pokemon dream rader which is in 3DS.Source
Hope this helps.