The Heron Moon

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.

Once you’ve started casting a spell as Colossal Dreadmaw, it’s too late . . . for an opponent to prevent you from doing that by killing The Colossal Dreadmaw.

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.

Official text for Emrakul, the Promised End can be found using the Gatherer card database at Gatherer.Wizards.com.

Official text for Colossal Dreadmaw can be found using the Gatherer card database at Gatherer.Wizards.com.

If a player is instructed to exile cards from their library “until” a certain event occurs, that player exiles those cards one at a time. If that player is an opponent, The Heron Moon’s ability will trigger that many times.