Nashandra

Location
Throne Of Want, after defeating the Throne Watcher and Defender, and having the Giant's Kinship in your posession.

Strategy
Nashandra specializes in dark magic and will immediately summon 4 Curse "fountains" once the battle starts. She has 2 methods of attack in addition to inflicting Curse on your character and eating away at his/her HP:

Scythe - She will swing her scythe twice or thrice (the third having a slightly longer windup and dealing heavy stamina damage on block), or do an overhead swing with a long recovery time.

Dark Magic - She will either fire a beam which you can easily avoid by strafing, or execute a small AoE attack similar to Wrath of the Gods. If she begins casting and her body is NOT being covered in dark energy, then she will use the beam; otherwise, it's the AoE attack and you must back away. If you look closely, you will also notice that her hand movement is different for each attack.

For melee characters it is also possible to equip light armor and dodge her attacks, ignoring the Curse buildup and only healing when your health starts draining.

Summoning help will make the fight easier. even if it's only Vengarl and Benhart, as they will draw aggro, leaving you with a good amount of room to deal damage and heal as needed.

Lore
Nashandra originated from the abyss and was originally the smallest fragment of humanity following Pygmy's demise. For this reason it was the first to become conscious and for the same reason, it was the first to want what it did not have. Nashandra later warned Vendrick of the Giants return, defeated them, and inherited the throne when the old king went hollow. There, she covets the first flame and the great soul.*

*This means that she wishes to have the first flame and the great soul for herself. The first flame refers to the first flame in Dark Souls I. The Great Soul is currently unknown.