Text (MT): Choose a creature type. ~this~ deals damage to target attacking or blocking creature equal to the number of permanents you control of the chosen type.
You choose the creature type during resolution. [Morningtide FAQ 2008/01/15]
Any spell or ability used to prevent damage would have to be played prior to resolution, which means prior to knowing which creature type will be chosen. [Morningtide FAQ 2008/01/15]
