The toughest part of being a boss is to fire people. But also as boss we have an responsibility to develop people. If you are not going to tell her what her deficiencies are how do you expect her to solve them. Try corrective action programme. First stop thinking that you want to fire her. Give her a letter explaining where her behaviour or performance is deficient, what she needs to do to improve , what the organization will do for her(training) and time farme given to her to show progress. If still same problems exist then you are fire her.
But be sure you deal with her fairly not on your predisposition to fire her immediately.