I agree with Noak.
Also:
1) It might be nice if the challenges gave players experience in countering specific deck types. For example, vs. an AI that tries to kill you with direct tower damage (encouraging players to counter it with cards like lookout towers), vs. a rush deck, vs. a plague deck, etc.
2) To support 1, it would be cool if the challenges could have special modified AIs. So, in the first example, the AI would be modified to have tower damage as its main objective, over wall damage. In the second example, the AI would give high priority to damaging cards, and low priority to construction and facilities. In the third example, the AI would give high priority to reducing opponent's facilities.
3) I definitely think things should be stacked in the AI's favour, since players can conceivably try as many times as they want, and they can craft decks to specifically counter the AI's deck. I like the idea of giving the AI boosted starting facilities and stock, for example.