First make sure you have pleanty or room for your birds. Space, space and more space.
chickens that have lots of interesting stuff to peck and scratch through pick on each other less.
Also put "stuff" in the run so there are obsticles extra roost bars and vision breaks. Make it messy, so there escape routes and places offering safe zones.
Offer food and water from 2 or 3 different places so low birds are not kept away from those resources.
I would stop separating the low bird. It just makes her more of an outcast. She already has no self confidence and separation makes it worse. Try putting her with just the rooster and see how they act together, then add back the other hens one by one. Separating them may humble them some.
Remember that chickens are the closest living decendents of t-rex. Pecking is comunication and thats not going away.
Thats why commercial egg factories trim the beak off their hens or they would turn canibal.
Good luck!