Typically cards with a limited effect scope don't work out too well. But somehow this actually works out decently. I dont think it would have been so if it didn't add bone shields in the absence of a preexisting shield. If I may make one suggestion, I feel it should simply play a Bone Wall card if there is no current shield. It's a slight overall buff and gives a few more uses while maintaining its current uses.
My only concern is how easily it's killed atm.
Though bone itself is not necesarily hard, typically in archetypical bone creatures, that layer of bone provides an incredibly dense and powerful defense. I almost feel the atk and hp need to be swapped.
