The best way to introduce new birds into a flock is the separation method. Birds don't take kindly to new members coming into their flocks and usually it gets pretty ugly when they are thrown together.
I have had good luck with this method...I take the new birds and keep them on the other side of a fence so that everybody sees everybody. but nobody touches each other and aggressions are worked out behind the wire.
Leave them this way for at least 3 weeks. Then when release day comes, the new birds are not actually new. There will be some pecking order stares and chases, but rarely does any blood ever shed.
So you need to keep her with the others, but isolated. Everybody needs to see her and see them. Put the original flocks water and feed near this new one so that there is a lot of close action. Leave everybody this way for a few weeks.
Then when you do release her, hopefully it will not be as bad as the first time. And yes, intervene if you have to. If somebody is getting really violent, remove the aggressor and cage her for a few weeks till the order can re establish itself and the new bird can get worked in.
Then when you release the aggressor, she will have lost her position and not be as mean toward the new bird.