The bitch in the cape with the sword?
If so this one's a bit tricky, there's 2 ways to defeat her. The first is to just lay a smackdown on her, this it what I did and it's the way to get her to drop her items (yes, she has 2 I don't think the second does anything though). The second is to break all the crystal containers in the room, there's 5, and then the big crystal up top, she'll flee if you do this. However after you've broken a few crystal containers the tall white flowers will start spawning in the room, if you get her grabbed by one of these (kite her) she'll end up horny and then if you lose you'll get the scene and the CG. Make sure to hit the confirm button before it finishes as once she disengages the grapple she'll lose the horny status and not get it again unless maybe you've positioned REALLY well? IF you don't you won't get the CG. It's the only futa on futa CG too!
Yeah was rather hoping at least on of the witches could turn the tables on her. Moreover it would've been awesome if ALL bosses were defeated the way you defeat the postgame boss, by rapin 'em!