Play through the game until you get to the Mercy Cellar and rest next to this area's boss room.

Now instead of going to the boss room left, go to the right. (With the key you'll need for the boss room)

This room is filled with enemies. Now use the Ashen incense (Also found in this area, at the very bottom of the map, hard to miss) and watch the whole room explode.
Now just keep resetting the room by re-entering and rest whenever you run out of spell casts.
That's it!