Stabbing in the back
Don't react immediately, think through your response. Don't ignore the behavior. Confront cautiously the backstabber about her deeds. Plan and practice carefully what and how you are going to say. Share the evidence of her behavior. Indicate the consequence if she continues in a non-threatening way. Avoid upsetting or offending her. Be prepared for denials. Maintain your high level of work performance, take care of your emotional well-being. Alternatively, befriend the backstabber and her allies. Let her vent about the problems she has to feel herself closer to you. Make favors for her ensuring next time she choses someone else be the target.