I didn't know what to do for a bit because her automatic rewind upon death would make her only able to be defeated by someone who can sense and attack her at any point in time she could retreat to, and achieve a death blow that way multiple times until she runs out of energy. Considering how overpowered she already is that would mean the list of people who could actually defeat her would either be 0 or close to it. The wizard antagonist of this game can only defeat her because of his super powerful magic poison that can limit her magic, but otherwise you're doomed.

Therefore, I have now come up with one method to stop this absolute OP ability and still allow her to rewind upon death. Granted it's still OP, but not to the point of ridiculousness as the previous method of killing her. There's a rune on her body (taking your advice) and said rune has a time delay of 3 minutes before activation. You can see the rune like a clock counting down. If you have magic control you can touch the rune and manipulate it so that it slows or even stops the countdown, keeping her dead until you decide to unblock the rune. Luckily no enemies can do this technique here so we don't have to worry about this breaking the game. Also, she can only go back in time at a maximum of 24 hours (so you can know where to find her again if you just keep watch on all of her previous locations), and due to the limitations of this roleplay from the poison she's still not able to go back further then when she woke up in the cell. 

The roleplay cannot be won without her surviving, but that doesn't mean you can't use the 3 minutes to explore further or other stuff. If you guys have nothing to do then just say so in the OOC and we'll just say those 3 minutes went by and go back to the cell.

The same can be said if we get through sections without casualties. Our characters would still have to go through everything again but we'd just glance over the stuff already done such as saying, "we got passed the monster again, and then..." and get right back where we left off. Why can't she just go back to where we left off? That would make things too easy if we can start further along back at full strength. That being said we'll just say she can't concentrate enough on her magic now to pinpoint her rewind.