Sorry for a second post but i have another suggestion...
Since there are only 24 spells, and multiple spellcasters, why not have a specific area on your interface, along the left hand side for example, with buttons to shortcut to your spells you already know.
After clicking the button, a ring would show indicating the area your spellcasters can cast upon, if they only have a limited range, and if you target outside the ring, the nearest caster will move to cast on that locale.
P.S. Still rootin for that hero!
EDIT: Maybe we could have it so that if your mage is in range of a spell he can counter, he will automatically cast the countering spell.