Question about magic sunders

question about magic i understand it sunders break immunities for a certain damage type but also give a penalty for said damage type…like a lightsunder 70 gives minus 70 on light question now is:how does this work with magic sunders?i cannot stock up on magic resist charms because there are if a hammerdin picks a magic sunder does this mean every hit by a magic source like…bonespear kills him instantly because he has no defense against it and a huge damage malus because of the sunder?