Think I've got any chance to win? (Pokemon Diamond)?

Considering your conditions, I would probably turn back. It's pretty amazing how you could beat the Elite 4 at those levels. I went in at lvl 55, and it was still hard for me. Since you're here though, you should try taking her on. Even if you lose against her, you'll have a feel for what she's like, and you'll be prepared for next time. If you do lose though, don't save.