You spot her right before she leaps at you.

What was she? Your eyes dart around the room. Suddenly you feel her presence above you, you glance upward just in time to see her form descending down upon you. By instinct, or maybe luck, you flick your blade upward right as she comes down upon you. Your blade slides deep into her belly as she becomes impaled fully upon it.

You are both on the ground and she coughs up blood as you slowly get up and look at her. She looks up at you. "Ridiculous... how could you do this?" Her voice is filled with shock, indignation and actual weakness.

Wow, had you beaten your first Monster Girl? You grab the blade, the same one that is still impaled in her. You could pull it out and slice her head off, or you could pay her back for the ****. Hell she made you lick her feet, you could shove your own feet in her face and demand she lick! Of course Monster Girls were tricky and powerful, maybe it would be best just to play it safe and behead her.