How to get a dementia patient to bathe?

I am taking care of my fiancé’s grandmother with dementia. She wears the same clothes everyday and goes for weeks without bathing or washing her hair. When she actually does “bathe” she washes her hair in the sink or wipes off her arms with a baby wipe. Her skin is turning brown and the odor is indescribable. She has violent tendencies and whenever I mention her bath time she says she already got one within a day or so OR she lashes out and throws something. As her caregiver I am trying to take care of her but of course “she doesn’t need help” because “nothing is wrong with her.” I feel as though her children are noticing she is not clean and will turn to me asking why... I’m not sure what to do because she has not taken an actual bath or shower in YEARS and I have only been taking care of her for a few weeks. Welcome to advice on how to handle speaking to her when she doesn’t listen and hasn’t ever listened to me even when she was in good health.