Ask about any weaknesses she has.

"Do you have any weaknesses an ordinary human could exploit?"

"What?" She narrows her eyes at you, so you quickly explain before the situation gets out of hand.

"You don't have to tell me all of your weaknesses, but if you tell me even one of them it'll be a good show of faith. And I'll also have some sort of fighting chance to make you keep your end of the bargain if you try to turn back on me. It's the easiest way to fulfill the first condition."

The demon hesitates for a moment and you wonder if you have to try to come up with something else... But she nods in agreement. "Alright, fine." She runs her fingers through her hair as she explains, "It's difficult to actually inflict any lasting damage on me through regular means. It's to the point where your kind all but assumes that I am immortal. However, the blood of an individual with certain qualifications can harm me."

"Like... A holy man?" You guess.

"A virgin." She corrects. "The blood of a virgin can halt my regeneration abilities and will allow even an ordinary man to kill me by stabbing me in the heart."

"What about the, er, hymen? Girls bleed with their first time."

"They don't always bleed, but to answer your question the blood that comes out during a sexual act does not harm me."

You nod like you understand the logic behind it, but really you don't. "I just have one final worry that you need to address... Now that the secret's out, how do I know that you won't just forcibly lay with everyone in this castle anyway to make sure I won't have any virgins to get help from?"

"It's simple. I cannot have sex with virgins without their consent."

"Eh? You can't?"

She shakes her head. "They're not invulnerable; I can still harm them with magic. But I cannot take one's virginity without obtaining some form of consent first."

In short... You can actually do something about her, right now. Perhaps the demon just doesn't know it, but you are still a virgin. If you act now, you might be able to banish her back into the mirror without having to even take on this bet of hers. And rather, the minute you agree to the bet you're basically going to try your hardest to lose the one weapon you already have on hand against her...