There is no such thing as Regigigas Sky Forme so there's no way you can get it. The Sky Forme is a myth.
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No... there is no regigigas in Pokemon Mystery Dungeon Blue Rescue Team. Do you know why? If not... I will tell you. This is because regigigas is one of those new Pokemon from the sinnoh region which was not invented during the time of Pokemon Mystery Dungeon Blue Rescue Team. However, I believe you can find a regigigas in Pokemon Mystery Dungeon Explorers of Time, Darkness, and Sky.
You can get Shaymin in sky form only in Pokemon Platinum Version.
Regigigas is a Sinnoh pokemon. Not a Hoenn.
You can't change Shaymin into Sky Form in Pokemon Diamond (that includes pearl). You can only change Shaymin into Sky Form in Pokemon Platinum (that includes Heart Gold and Soul Silver). You can get the Gracidea to change Shaymin into Sky Form.
It changes back to Land Form at Nighttime and It goes into Sky Form in the Daytime. If you don't want Sky Forme at all then start over without a Gracidea Flower.