I agree with aart, more info would be helpful.
From time to time, I've had to cope with a bully, and it's not fun trying to figure out why they behave that way, and it's so sad to see the others terrorized.
The first thing to try is to remove the bully from the flock temporarily so the pecking order is forced to shift. Then when you bring the bully back, her rank in the pecking order has changed, hopefully, and she won't be The Queen. Also, a week of "time-out" can interrupt her bad behavior and she may forget about the joys of terrorizing the smaller chickens.
Often, increasing the space in the run can go a long way to resolving bulling issues. If you add tree stumps and other places to perch, having places to retreat can help.
When that fails, there is pinless peepers to try. It will alter the forward vision of the bully so she can't see her target as well.
If that fails, as all of the above have in my flock with one persistent bully, you may need to create a sub-flock and give the bully and her cohorts their own space where they won't disrupt the rest of the flock. This has works very well for my flock. It required adding a run onto the one side of the coop and partitioning that coop off from the others so the bully group can't start fights and throw their weight around.
Or you can simply eat the bully.