Oh God, why did it have to be a giant spider? If you can stomach to the process of reaching this boss, you'll find its mechanics are pretty straightforward. Beating it, however, takes a bit of practice.
When you first enter its lair, you'd do well to wipe out as many as the little spiders as you can, lest they catch you off guard. From there, merely dodge Freja's attacks and smack it in one of its two faces. Watch the video above for more info on the encounter.
For more help be sure to check out our complete Dark Souls 2 Boss guide.