The very first Mirage Spot I found was Cresselia's. I have been trying to soft reset for a perfect 5IV, but it is taking a while. I saved the game on Crescent Isle, but I am afraid that if I leave it to challenge Victory Road, I am uncertain of whether or not the Mirage Spot will remain.
They way Mirage Spots work is: Every 24 hours a new Mirage Spot appears and the old one vanishes (hence the name). The only way to get more than one Mirage Spot at once (Note: Pathless Plain and Nameless Cavern do not count, as they are permanent until you catch their respective trios) is to obtain one via StreetPass. These secondary islands will also disappear every 24 hours.
If you save the game and power down while on a Mirage Spot, it will not disappear. However, as soon as you Soar/Fly to another location, the Mirage Spot will disappear and a new one will take it's place.
One final note: There is only a 5% chance of Crescent Isle spawning on its own, a little higher with StreetPass. Congratulations on it being your first Mirage Spot. Kepp trying until you get that Cresselia (unfortunately, you can't really check it's IVs after a day is up)!