The Heron Moon

If you choose to cast a card as Colossal Dreadmaw, while on the stack, treat that spell as though it were Colossal Dreadmaw (including the mana cost to cast it). After it resolves, treat the permanent it becomes as though it were Colossal Dreadmaw for as long as it remains on the battlefield. Once it leaves the battlefield, the card goes back to its normal self.

The third ability only triggers once for each instance where cards an opponent owns are put into exile, no matter how many cards were exiled at the same time.

For many playtest cards, you’ll need to make a generous assumption that basic game rules would be updated to allow them to work. The Mystery Booster 2 Playtest Card Notes section (reproduced here in individual Gatherer rulings) provides guidance for fitting these cards into the existing rules structure.