The best Legit way of getting mythical Pokemon is:
I've had many friends help me out when they had many copies of, say, Genesect lurking in thier PC.
Although it is noted that: You can get mew by glitching Red/Blue VGC (if you count bug abuse as legit), You get Celebi in Silver VGC Legit, you get Deoxys in ORAS Legit, and you can get Shaymin/Darkrai In D/P (again, bug abuse, but if you do this one wrong, you may corrupt your save file).
Hope I helped!