I loved how the tactics would creep on you. There's the obvious, but then there's the first-mover advantage and the idea that you can force the demon to place effects on itself and the like. I would have liked a clearer game plan for the demon. Sometimes, it would hurt itself even when by playing another card it could have chosen to have no effect. I feel like the ability to reason out the demon's moves would have really taken this to the next level.
I will say, it didn't quite seem to fit the theme.