So I am planning to use Shaymin-Sky in the 2013 Winter Friendly tournament, which means it have to be in the battle box. But as soon as I put Shaymin-S in the PC, it reverts into the land forme. What should I do?
Shaymin-S will always revert back to land form when it is in the PC, so there's no way you can use Shaymin-S. You're going to be content with regular Shaymin.
Source: Experience