This can be an annoying fight and you'll want to stay on horseback for it. Once she is free, you need to run parallel to her and attack with your sword to smash off her muzzle. Don’t get in front of her. Once it is off, she will call a few of the really small enemies. Just shoot them and watch out for them coming from the sides. Once they're dealt with, go back to the Stygian. Just like that little one before, keep shooting and she will go down. Use boost if it’s getting too close.