Acid Rain

You wouldn't be able to get more than 1 out at once.

Unless you have a double battle, one Castform on each side, a pursuit user on one side faints the other non-Castform, and then a third Castform is sent out activating Acid Rain.

But then only one Casform will transform at a time. They won't simultaniously do it. They'll take turns transforming, but since one of them will never finish, the others won't get a turn.