Priscilla can be found at the end of the Painted World of Ariamis, which is an optional area of the game. You do not have to fight her to exit the Painted World, as you can just walk by her to the exit. If you want to fight her though, just hit her once to make her aggressive. Once she becomes aggressive, the first thing she will do is turn invisible. Cutting off her tail is not easy, since she's invisible for the beginning of the fight and it's rather small compared to all the other tails you can cut off. As said before, as soon as you attack her, she will turn invisible. The trick to getting hits on her is using the snow on the ground as a means of knowing where she is, since her footprints could be seen in the snow. That means you should always stay on the side of the area with the snow on the ground; when her footprints are getting close, move around to the back of them and start attacking. Three good hits in succession with a decent-damaging weapon should make her visible again. She only really has two attacks; first is her swinging her scythe across her body (does bleed damage), and the second is some kind of frost breath ability that spans in front of her. As long as you stay behind her, neither of the two attacks should hit you.