@Lacy Duckwing -- I have used the method of isolating the timid bird from the flock and it does boost her confidence to where she's returned to the flock & doesn't pay attention to the bully. If a timid bird continues victim behavior other birds will join bullying. But once the timid bird is isolated for a few days she returns to the flock unafraid. I've noticed sometimes a broody bird gets picked on by others so I isolate broodies too till they break the broody behavior (I don't have a roo or raise chicks so when a hen goes broody we break her to keep her from getting picked on). Birds can be mean to each other -- just like mean bullies in a children's playground!
You may not have room in your run to temporarily fence off one corner, but you might be able to find a corner in the coop for it. Or perhaps you've been meaning to expand your run and just need some motivation. This is a good time to take a look at your setup and think about pushing out a fence to make 100 more square feet of space to make your flock happier. In a pinch, a dog crate large enough for the bully victim to stand up in can serve as a safe refuge during bully victim treatment. It only needs to be used during the day as the victim will roost with the others at night.