It triggers when the spell is cast, so it becomes a creature before the spell resolves. A creature that would destroy an enchantment when it enters the battlefield can't be used to destroy this card, since it will no longer be an enchantment when the creature enters the battlefield.

When it turns into a creature, it is no longer an enchantment.

It changes into a creature even if the spell is countered.

A "creature spell" is any spell with the type Creature, even if it has other types such as Artifact or Enchantment. Older cards of type Summon are also Creature spells.

A noncreature permanent that turns into a creature can attack, and its abilities can be activated, only if its controller has continuously controlled that permanent since the beginning of his or her most recent turn. It doesn't matter how long the permanent has been a creature.