OP mentioned they free range with the broody.
I would try the anti-pick spray or I would separate out the aggressive chick for a time out. Keep it out several days and then try putting her back out and see how things go. I'd remove the aggressive one, not the one being attacked. If you take the one being attacked out you will have a much harder time getting it re-integrated. Pull that aggressive one, give her a good time out for a few days and see if she has an attitude adjustment when she goes back.
I would try the anti-pick spray or I would separate out the aggressive chick for a time out. Keep it out several days and then try putting her back out and see how things go. I'd remove the aggressive one, not the one being attacked. If you take the one being attacked out you will have a much harder time getting it re-integrated. Pull that aggressive one, give her a good time out for a few days and see if she has an attitude adjustment when she goes back.