When you meet up with Farah again after the brothel, there will be an intermission sequence in which Mahasti will attack you. Climb up to the save fountain, then down to a platform where Mahasti will be waiting for you. Use whatever fighting tactics you prefer. If you get knocked off the platform, rewind time and dodge the attack. After awhile you will transform into the Dark Prince, and Mahasti will run away. When chasing her, always run to the right, kill the enemy on the platform in between to fill a sand tank, and heal. Then, when you grab the ledge of the platform Mahasti is on, slow time and jump up. Slash at her until slow time ends and she jumps again. Repeat this until her health is depleted. You will enter a deadlock that you will have to win. If you lose, Mahasti will jump across the arena, and you will have to try again.