Demon Wall can't be attacked from behind either can it?
Even assuming that no rear attacks will be possible at Levi, you still have flank attacks.
Since we don't know yet if the head will over over the edge of the platform so you CAN get rear attacks in or if you'll be able to attack Levi's tail instead or basically anything, it's a little early to do more than be concerned... Complaints are for when you have more facts.
Edit:
If both tail and head is attackable and tail provides the rear/flank attack positions... wouldn't that be the sort of new mechanic in boss fight that people have been crying for?
