This battle was a close one for me. This boss exhibits strength in numbers. If you're not careful, the sheer amount of enemies on the field can overwhelm you.
You first face off against three giant fire magic wielding skeletons. They take a long time to actually start hitting you, so use this to your advantage to get in one or two hits on of them. Use the mounds of skeletons as cover, as it can be hard to tactically maneuver the field with three enemies attacking. If you think three is bad though, you're in for a treat.
After X amount of seconds pass, a bunch of smaller skeletons will begin to spawn. The lives of these smaller skeletons counts towards the boss health bar, so you need to eliminate every last skeleton in the area.
Move around the perimeter of the area, using the 4 mounds of skeletons as cover, and finish off the enemies one by one (or two by two if possible).
Once you beat this boss, you will get 'Skeleton Lord's Soul' and 15,000 souls