Actually, depending on how its built, a ballista is nothing more than a very large, immobile crossbow firing a 3-foot (Even though it's exaggerated in this game) bolt with enough force to smash plate armor. If you got the right shape of stone and fitted some sort of cup or holding device to the string, then, yes, a ballista could fire a stone (Albeit very inaccurately). But that's not its purpose.
What I'm saying, however, is to focus entirely on researching longer-reaching means of using magic. Generalize your knowledge, try and get an overall bonus out of the deal here. THEN, once you have that basis, revise specific spells and items for use at long range. This should provide overall bonuses. The further back you stand, the less you die. After you gain this FUNDAMENTAL advantage, you get into the Dirty Tricks Department (See below)
And when I say backfire, I mean change fundamentally. Fly in a different direction, explode BEFORE leaving caster's hand, or, if our roll fails, turns bright (Insultingly bright) pink and kills EVEN MORE of our guys! IT'S GENIOUS!
And it might be time to start looking into magical subterfuge. Can Arstotswhatsis read minds? If not....time to infiltrate. Because once you have friendly magic-users who have learned the fireball spells/wizardry and are in their lines, you start disrupting other mages spells. Then, in the confusion, the allied spy zaps a few hundred Arstotzkans, and says "THEY ZAPPED ME WITH A CONFUSION SPELL" before either A. succeeding brilliantly or B. Dying a noble, hopefully flammable, death involving many dead/exploded Arztopskins.
Won't be long, if you start infiltration, before they can't trust each other, can't trust magic-users.......See any possibilities? *Evil grin*