Dark Souls

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.

It took me a couple of tries to get this method down, but there is a straightforward way to defeat Priscilla, earning her dagger and not resorting to specialist spells or equipment.

Equip the lightning spear found on the mimic in Sen's Fortress (I would imagine it's almost impossible for players to get to this point in the game without having picked up this weapon) and use your best shield with your left hand. Light armour for maximum dodging is also recommended, but not essential.

Priscilla will turn invisible as soon as you strike her (or immediately if you're returning from a death or reload) and the only way to track her is by watching for her footprints in the snow.

Opposite from the doorway where you enter her arena is arch leading to a few stairs and a sheer drop. Run to the top of these stairs, face the arena and watch for Priscilla's footsteps below you. Once she walks through the arch, put your shield up and run forward - when you hit an "invisible wall", you've actually run straight into her, so spam your spear's light attack. A flurry of light attacks seems to be better than one or two heavy attacks to stagger her and cause her to become visible, whilst causing very little damage.

Once she's visible, move into the centre of the arena so you can circle around her and strike the tail for the dagger. Priscilla is a very easy boss to dodge and has very low health, so it's important that you concentrate on detaching her tail for the dagger she drops. Again, light jabs with your spear are the best way to stay accurate without causing too much damage.

At some point, she'll become invisible again so run back up the stairs and repeat. I personally got the dagger during the third cycle of this, by which time she was down to 40% health. Finishing her off by dodging attacks and using the spear was pretty straightforward, but you can use your own heavy hitting weapon or spells if you wish at this point. If her health gets too low before obtaining the dagger, you can start again by quitting to the main menu - this will place you outside the fog wall when you reload.