Can you afford to get one of those dog cages from Lowes (about $65 plus tax)?
They are large enough for one or even two full sized hens to live in temporariliy.
I have one, and I just keep it in the main henhouse 24/7/365.   When I need to isolate a chicken for some reason, I put the chicken in that cage, where she can still live among her flock, but she is protected from danger.
In a case like yours, I would isolate the victimized hen in that dog cage for a few days.   Of course, I'd provide her with food and water -- and maybe special treats like some suet or something to help keep her entertained.
But then after she healed a bit and got some good food and water into her belly, I'd turn the tables and lock up the main bully(ies).   Even if I had to cram in three of the worst bullies into that one cage, I would do it.   Offer her freedom with the better behaved birds, while the biggest behavior problems got several days of lockup.
I had one lady who was shaping up to be a particularly bad bully, and it took an entire week locked up in that cage, but I got her behavior under control by caging her that way.