Getting the Mooshu spiral key has nothing to with what level you are. You need to complete the main storyline quests in Marleybone in order to get the key. The last set of quests you must complete are inside the museum: Counterweight East, Counterweight West, Big Ben. When these are completed, they will tell you to report to Headmaster Ambrose (in Wizard City) who will then give you the key to Mooshu.
Best if you are level 22-25 (this is where the normal progression should set you as you tackle Big Ben)
If you know your character and how to play the school, it isn't bad. If you mis-build your deck, or don't realize how to play your character, it is a nightmare. Just remember, there is healing on like the 3rd floor, and you can go back in this dungeon without exiting, so you can refill after each battle.