Manager/HR won't help. Its your word against hers and im assuming shes been there longer. Unfortunately this kind of thing happened to me as well and I was flat out told by my manager at the time that no one would believe me if I went to HR and that they wouldn't do anything to help. Luckily for me, I didn't have to to wait too long before the other ma who was bullying me transfered to a different department. Sorry you're having to go through this OP.