This is one of those few terrible design decisions in this game.
I still havn't seen frostwall yet. I can't respec my warrior due to not being able to get the aoe skills again either. Sucks.
Other than that its a great game. Just make some books more easily available.
Mage spells tend to be craftable. If you get a scroll for frostwall (from crafting or adventuring), then you can put it into a skillbook that is also craftable (or as well can find or purchase).
The recipe for crafting water elements has changed so its harder to produce them in quanity. Obviously u can save scum if u really want it, and don't have enough mats.
Edit:
http://www.larian.com/forums/ubbthreads.php?ubb=showflat&Number=511616Icewall needs crafting rank 3.