As soon as the boss starts, the first thing it will do is shut out the lights. You should either turn up your brightness settings or, before the fight, light some oil places on fire that are located up the two stairs that are visible on the way to the fight. Spells are not recommended for light, as they will burn out eventually and the same goes for torches. The boss itself is pretty fast and agile, reminiscent of Artorias from the first game's DLC, though not quite as hard hitting or aggressive. The best tactic is for at least a single summon to help. Whether it be for aggro bait or simple to take pot shots to whittle down her health.
Note: With the lights off your lock on range is greatly reduced making it difficult to use spells. With the lights turned back on by burning the oil in the two adjacent rooms before the boss fight, your lock on range is returned to normal making spells easier to land.