I don't know if this is intended or not as Fireball has a 3 turn cast (maybe its a typo on the spell details?).  My fire mage hero had the skill that gives you +10 initiative so that he could get off 2 fire darts spells at the start of his turn.  I had also picked up a staff that gave a mastery bonus (can't remember what it was called).  When I learnt the fireball spell I found that on his first turn I could cast it in a single turn consuming both my original actions, i.e. 2 turns not 3 to cast.  Since the AI hadn't moved yet, my mage had the highest initiative, and was all bunched up I could often kill everything with a single cast of this area effect spell.  Great fun but a little OP.