Question

How to get a lugia with multiscale?

Answer 1

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

Comments

Actually, only when SoulSilver is inserted in the 3DS. You may want to change that.

Answer 2

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.