You answered this in your first sentence: she has dementia! That’s why it’s happening. In good news, these things pass. Here’s what I would do: find out if there ARE things you can adjust in her environment to make her more comfortable. Add music for calming, see if she’s on medications that could be making her more aggressive, add some extra snacks and water throughout the day, make sure her clothes are comfortable and fitting well, all of that good stuff!