To balance, I would actually suggest always having attack powers lose 1 atk (instead of negative attack values gaining 1) and continue to go negative, since negative attack values are the only ones that can hurt you. Over time, the shield would actually turn against you as creatures' attack values become negative, making it a double-edged sword if not watched and taken care of. I like your idea, but I don't think it's balanced the way it is, forcing attack values towards 0.