Question: "My direct supervisor thinks I'm doing great, but there's another manager who has suddenly turned on me. She attacks me in meetings, humiliates me in front of others, scolds me in email messages and copies them to other people, etc. I've seen her style -- every couple of months she pressures someone until that person quits or she fires them. I've spoken with my boss, but she is sort of his supervisor and he can't protect me. I'm feeling worthless, paranoid and upset at being forced out of a good job. Can scapegoating be prevented? Once it starts, is there any way to turn the tide? Is it best just to leave?"
Answer: I've seen people win at these situations just by toughing it out. She will get tired of going after you and eventually go after someone else. And eventually, she will have done it enough that she'll go under.
You already have a intellectual handle on this because you know it has nothing to do with you. She does it because that's what she's like. She probably also does it to people who make her feel threatened, which is a compliment to you. She wouldn't bother unless she knew you were good and my guess is that she actually knows more about how good you are than you do.
Here's my advice. Don't quit. If they are going to fire you, make them do it. Often you can get a nice severance package that way. But start looking for a job right away. Just getting out there and realizing that you have value will make you feel better. Your focus for now should be to consciously do the things that raise your self-esteem. Maybe it's volunteering. Maybe it's a great new outfit. Maybe it is exercising more, or eating better or going to church. Maybe it is all of the above. You are in charge of how you feel. The best way to win against the "hers" of the world is to choose to treat yourself as if you have value.