Yeah. You are about half way through the game. I agree that Sanctuary keeper is hard, but if you do it correctly, it is cake to kill. The boss after that however, is MUCH harder.
**Spoilers**
Her name is Yunalesca and she has three forms. The first is pretty easy, the second is harder, but still pretty easy depending on how strong your guys are, and the final form is just plain hard if not very hard.