Impervious is a good card. It'll block almost any attack in the game for you by itself. Rather than tell you obvious things about why it's good, I'm going to mention why you might want to be wary of it. The danger of making a deck too focused on defensive cards is that a lot of the enemies will get stronger turn by turn. If you aren't keeping pace with that, or ending the fight quickly enough, they can get out of control.
