Essence Backlash

• WWizards of the Coast2012-10-01
  Official Ruling
  Abilities that change the power of a creature (such as the one of Collective Blessing) affect only creatures on the battlefield. They won’t change a creature spell’s power on the stack.
• WWizards of the Coast2012-10-01
  Official Ruling
  Essence Backlash can target a creature spell that can’t be countered. If Essence Backlash resolves, it won’t counter the spell but it will still deal damage.
• WWizards of the Coast2012-10-01
  Official Ruling
  If the creature’s power is * and it has an ability defining its power, use that ability to determine its power.

Rulings via Scryfall