Before you fight The Lost Sinner--who has one of the four great souls--you'll need to do some prep work to make the encounter a bit more manageable. See, she starts off in a very dark arena, but there's a way to illuminate it! First, you need to kill the , whose pathway eventually leads to a dead end, where you'll find a key.
With that key in hand, you can unlock the doors on either side of The Lost Sinner's arena, and, using a torch, light the oil gutters on fire. Now you'll finally be able to see the boss. After that, it's just a matter of learning her attack patterns and dodging her strikes, which you can study in the video above.
For more help be sure to check out our complete Dark Souls 2 Boss guide.