Challenge of Ages

Choose a creature you control and a creature an opponent controls. They battle each other. If
either of these creatures is tapped, untap it. Both creatures may use any of their abilities (mana cost
for any of these abilities is {0} during battle) first;
then each creature deals damage to the other
equal to its own power. If either of the creatures
is destroyed as a result of this damage, the
destroyed creature's controller loses 4 Life.