This achievement is for killing Priscilla the Crossbreed in the Painted World of Ariamis.
You enter this area from Anor Londo, in the giant church. You'll come into this building in the rafters first while fighting gargoyles, and after crossing them you'll reach the huge spinning platform you can use to move up and down to various levels. If you move it down one level and go to the bottom, you can step into the bottom floor of the church.
The giant painting at the back can be examined to bring you into the Painted World, but only if you have the Strange Doll. This can be found by going back to the Undead Asylum:
-Start riding the elevator shortcut up from Firelink toward the Parish, and step out partway up
-Move to the right side grassy area, then roll forward to land on a big pillar
-If you don't have it already, grab the item you can see on the roof nearby after climbing up. This is the key you will need to get to the other part of the Asylum.
-Follow the pillar and stairs up to the crow's nest, press A to roll into a ball when prompted, then wait to be taken back to the Asylum.
The Doll is in the cell you started the game in.
Be warned that once you enter the Painted World, you can't leave until you make it to the exit. There is a bonfire right at the beginning and quite a few things to fight and explore.
To get to Priscilla, you need to open the big doors in the middle building that will lead toward her. To do this, you need to reach the courtyard area (from where you can open the big doors as a shortcut back to the bonfire) and go into the building on your left if coming from the bonfire. Go downstairs into a really dark room with several of the wheel skeletons, and there is a crank you can turn in there to open the door.
Once you have, you can walk all the way to her (with some enemies in the way) and she will ask you to leave. If you attack her, the fight begins. She will turn invisible, so watch the snow to see her footprints and anticipate her attacks. She is using a scythe, so she can hit a wide area and fairly far away.
Once you hit her a few times, she will appear and be targetable. Get in hits while you can, and then the cycle will repeat. Make sure you hit her tail enough to get Priscilla's Dagger prior to killing her. Once you have won the fight, you can leave by walking toward the ledge at the back.