Much like the same I said from my Preemptive Strike review, but the same principle applies:
The rules change to Defense makes this card much stronger than it looks. It can make you a Defending character long after the basic defense window is passed, but the real power of this card lies in its power to save a Tough card. Effectively, you make a basic defense when attacked by the Villain. This lowers the Villain's attack by your defense. Defiance takes out the Villan's boost card. What does this mean? It means if the Villain's attack is equal to or lower than your defense, its attack is zero. With no boost cards, it will deal no damage. Which means any existing Tough cards you have on you are saved for another attack! I find this card is great in Protection cards that run Tough, like Doctor Strange or Colossus!
I also run this with Tony, using Energy Barriers to deflect the Villain's base damage.