Opalchampion

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

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 can't be used to destroy this card, since it will no longer be an enchantment when the creature enters.

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

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 {T} abilities can be activated, only if its controller has continuously controlled that permanent since the beginning of their most recent turn. It doesn't matter how long the permanent has been a creature.