If they went with a resource pool (like mana) for spell casters, it could work kind of like ciphers. You have a max amount (say 100) and you start each encounter with 1/4 of your max (so 25). Your mana would slowly accumulate over time. Spells would cost say 10 mana for each level of the spell. You start the battle by deciding if you want to cast 1 level 2 spell, 2 level 1 spells, or wait and let your mana increase so that you can cast higher level spells. Of course just like the per encounter system, the difficulty balance and using the same spells every encounter depend on how they balance the spells and enemies.