What Yewstance says is true for the most part, but I think he vastly underestimated the spells.
Athnul can learn to properly use spells in both roles and besides Cure (which you automatically get back every time you banish it pre-role, since it's the lowest AD spell)
Three months late on this response (I haven't checked the ACG forums in a long time due to lack of new material), but it seems like a couple people in this thread are doing this wrong. And I could have sworn there was a discussion on this before, but I found nothing from a search.
Athnul can't put AD3 Cure in her deck pre-role*. The adventure card guild rules specifically say you can't have a card in your deck higher than your tier. So choosing spell as a card feat prior to tier 3 isn't legal because you can't make a legal deck (unless you use an Ultimate deck in combination with her).
* Unless of course your tier 3 scenario offers a card feat as a reward (which I can't recall any that do) or after receiving your card feat at tier 3, you choose not to level up to tier 4.