Jonny (springer) doesn't like having his back end groomed and complains something awful. I have to put him in a kind of half-nelson under one arm while I do his bum and back legs so he can't bite me! It sounds really cruel, but it 's just being firm and assertive really. I always finish by brushing his chest which he loves so we finish on a positive note. Maybe you have to just be firm with someone holding her until you get the matts out, and then try to reintroduce regular gentle grooming. Have you tried the kong zoom-groom? It's made of rubber and while it doesn't get undercoat out, it does massage the skin and pull out loose hairs.
