I think it is a bit harder when you have one being picked on by all.
If you have a dominant leader that is doing most of the brutality, you can shuffle the pecking order by removing the alpha girl for a couple of days, can also take the 'top 2' out for about a week then let them come back in.
It should lower the bullies in the pecking order and give the little one a chance to assert herself, or at least put on some size.
There was a thread in the title area a couple of weeks ago about coop bullies, I will see if I can find it.