The Witches of Hemwick are the fifth boss you'll take on in Bloodborne, and they might be the easiest yet — if you move quick enough. Soot Monsters will spawn into the battlefield constantly during the fight, so you'll want to manage their numbers, but prioritizing hitting the Witches whenever you see them. You may need to upgrade your weapon or your Strength and Skill attributes if you're not doing enough damage — if you don't take out the first and second witches fast enough, a third will spawn. Also, you've got to kill them within a minute or so of each other, or else they'll all respawn.
It's not as tricky as it sounds, but you need to keep on the offensive. Check out the walkthrough video posted above if you want to see how we tackled it.