I love the idea, it fits entropy PERFECTLY, however I have a major concern. Its almost too random, think about it this way: You are playing a fractal phoenix deck against this shield. You attack with 10 minor phoenix's, you will have no idea what the hell just happened; which phoenix hit which shield. I guess that could be intentional, but the major problem I see in most crucible cards is that they are too complicated. This card seems VERY complicated not only to understand, but to code. I'm no programmer, but I'd put money down that this would be laggy. The code would have to generate a new shield every single attack, meaning each of the 10 attacks requires a new shield generation, which requires processing power, which creates lag.
My proposition: Make it a permanent card with the following effect:
Unupped: "Each turn your shield is replaced with a random basic shield. If you have no shield, a random basic shield will be given." 5
Upped: "Each turn your shield is replaced with a random advanced shield. If you have no shield, a random basic shield will be given." 5
Make sure you read the text VERY carefully, with the unupgraded you will always have an unupgraded shield, whereas with the upgraded you will first recieve an unupped shield, and if it is still in play at the end of the turn, it will be replaced by an upped shield. This would be much easier to understand, and adequately entropic, while still being very potent if used correctly. Also this would nearly eliminate the lag from the original idea.