Hello,
You could call your vet or local groomer to see if they can help. There are also mobile groomers that might be able to help with managing stress and getting a thorough de-matting. In older cats you have to be very gentle and go slow.. but I feel that the negative impacts of not grooming outweigh the stress of trying. Find someone who can help you locally and go slow. Please dont use scissors or over stress her. good luck