Lock your big girls either in the coop or out in your yard, put the chicks in the run. Let them run all over and explore. Then several times durning the day, go out there and give them a mock chase, so that they find the hideouts, the safety zones.
In several places, put up situations where as a bigger hen can chase a chick, but cannot follow. Beware of setting up a trap, where a chick can get caught away from an opening. Personally I like something just a few inches off the ground that can be approached from any side. Taking a pallet and putting it just a couple of inches off the ground will work. Lattice fencing will work, those white plumbing tubes will work, say a 6-8 inch tube.
Then the next day, late in the afternoon, let the big girls in except the mean one. Leave her out. Let the milder hens and chicks work this out for a few days. Then add the mean one in under supervision. By this time, the chicks will have learned how to get out of the way, and when to quit irritating their elders, and where good places are to escape. And they will be able to out run her.
Mrs K