If you want a Lugia egg, first get the actual one. Then get a Ditto and put the actual Lugia and the Ditto in the day care (located south of Goldenrod City). Wait for them to have an egg and you'll have a Lugia egg.
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You need Ho-oh and Lugia
it wil never come back...
Lugia learns Sky Attack at level 99.
You get an egg from Professor Elm's assistant from any Poke Mart.
Most Pokemon can, so I guess it probably can. If Lugia can, I bet Kirlia can. :D