Regarding the fact, how many enemies you can have in a fight, one or two times will be hardly enough. Anyway a higher cooldown could limit the amount of uses anyway.
I understand magic armor more like a shield, that surrounds you, so I guess, body parts would not fit there. The idea there was to split the shield into different elements: fire, air, earth, etc
But that would be perhaps to complicated. On the other hand you could just mixed two of the suggestion, for example: body parts for physical armor and penetration/treshold against magical.
