For mana spells like stunbolt, I think all you do is roll Willpower (plus Counterspelling)  to reduce the hits on the attack spell. Any uncanceled hits on the spellcasting test are added to the force of the spell as damage.<br/><br/>Edit-To clarify, Body and armor won't reduce the damage from mana spells, because they are dealt internally, bypassing armor. However, they can't affect non-living objects, either.