This is an old thread, last piosted on a year ago but....
It depends if the attacking hen is doing this regularly. It is not uncommon for hens to have squabbles, even quite aggressive ones and occasionally they lead to more serious injuries. This is all part of the chickens' social behaviour, establishment and maintenance of the pecking order, but if one gets particularly and persistently nasty, it may mean that freezer camp or re-homing is necessary.
It's worth a bit of 'boot camp' first...isolate the aggressive hen for a few days, but within sight of the flock, then see how she behaves when re-inroduced. It may just tilt the pecking order enough to stop her. If that doesn't work, you could try isolating her AWAY from the flock, removing her position completely. You would, however, need to keep a very close eye on her when she is re-introduced as it could go two ways....she could end up lower down the pecking order and therefore be less aggressive or she could become ultra-aggressive whilst trying to fight her way back to the top. I have had it work well, but I've also had it lead to a re-home.