The level is about medium-sized, and is not too difficult even above Normal. You'll go through a series of rooms, some connected by doors, some by paintings that shimmer (the camera will show them when it happens), fighting stuff along the way.
The boss isn't too painful on Hard, especially if you have some Mika cards for reviving. Just slice a lot and dodge (move the left stick + B), to get the powerful X counter attack. He has two stages, and both seem the same. When his health is gone, you'll get a prompt to repeatedly mash X to win a sword clash. After that, and when prompted (and with Blood in your meter to spare -- which shouldn't be an issue since you're not using it in this battle, and there's a Nurse to find on the balcony just before getting this far), hold RT and press X. Repeat the same fight a second time. The achievement unlocks after the cut scene, when the mission results appear.