Priscilla is a completely optional boss that does require some steps to access. First, you must return to the Undead Asylum (go up the elevator to undead parish, but don't let it go all the way up and go onto the broken roof. Then roll onto te pillar and up the stairs to the crow's nest. A button prompt will appear that says ' to curl up like a ball'. Wait about 30 seconds to be taken back). Once you're there, go back to the cell where you started the game and pick up the Peculiar Doll. Then go to the painting in Anor Londo and interact with it. You'll be taken into the Painted World of Ariamis.
She won't be hostile when you enter the fog. Of you want to just leave the place, walk around her and jump off the edge to leave. However, for the achievement you must kill her. Before the fight actually starts, I recommend getting your hardest hitting weapon and doing a two handed strong attack on her tail for her tailcut weapon. She will aggro and turn invisible. Watch the ground for her footsteps and keep your shield up at all times. When you here her swing, look where her footsteps are and attack. After a few hits, she will become visible again. Rinse and repeat for the achievement.