it is fine on the hero's as that doesn't give an ability to forever stall a fight.
Oh, I just noticed I didn't go over why I don't think the health drain on magicalGirl is a good thing: Because it sets the HP to 1 you have no other choice than to attack swordman until she heals. This means that until she heals your options are basically reduced to 1 in most cases. Technically you can guard spam until she heals but as I already said that probably shouldn't be a thing.
Another way to make guard less able to stall would be to have it cost more mana for the player so it actually costs mana to cast because right now you get enough back to cast it again the next turn.