We've got your back. With the Tough-O-Meter, you'll know whether to bring extra layers or Swiss army knives as you summit the literary mountain. (10 = Toughest)
(2) Sea Level
The Lion, the Witch, and the Wardrobe is a pretty straightforward book to read. The prose itself is relatively easy, with short sentences and simple vocabulary...although there might be a few unusual (or unusually British) words or literary references here or there that you need to look up.
The difficulty comes in the interpretive process. The main thing that you need to get is that the book is not only a great adventure story, but an enormous allegory for the central idea of Christianity: Christ's sacrifice to redeem the sins of others.