originally i would say,....kill her with kindness with ever ounce of self worth you can muster up.... ...but it sounds like this has gone on for too long.
beat her at her own game.
stick a knife in her back on even numbered days, and kill her with kindness on odd numbered days.
make sure there is always witness's on the "kind" days, and never any witness's on the "knife in the back" days.