Folks who are negative outwardly are usually at least twice as negative to themselves inwardly, so I would recommend that you try finding something (anything!) you can compliment her on, and tell her! I know it’s hard to do, especially with someone who feels so “toxic”, as you say, to be around, but this is an amazing exercise when you use it, both for her and for you! Often you can soften someone (even someone with the sharpest edges) by caring for them in some small way.
That being said, if she is doing real harm to you at work, be very honest about your boundaries, and continue to give her positive feedback along with constructive criticism when necessary. It is amazing how this works! I have used it in many situations, and not only can it improve a difficult situation, but it also offers you the opportunity to see someone in a new light once they begin to soften.