I have a similar problem with a silver laced sebright named Ruff. She still occasionally picks on a 2+foot tall brahma rooster. We got the rooster along with 3 other chickens as chicks when Ruff was a couple months old. At one point she was so mean that we isolated her from all other chickens(where she couldn't even see them) for a few hours and she was nice to the four chicks afterward. So try isolating your mean hen from other chickens for a day in an area where she can't see them and see if that works.