I think you need to focus on making the new hen more confident, I swear sometimes the victim almost causes the aggression.
So locking the flock out of the run, and the victim in the run can let the victim explore, eat and drink without being chased can give them some territorial rights. 4-5 days of this should help.
Is the run cluttered? Check out a cluttered run. Many runs are just wide open spaces where one bird can see all the other birds 100% of the time. Adding pallets, mini walls, roosts, feed bowls where a bird is out of sight while eating can help.
Pin less peepers on the aggressive bird can make a huge difference too.
Mrs K