How to defeat Vittorio and his ballista?

I would suggest using Cave Lords and a surprise attack. Then, when teleporting, the Cave Lords can destroy a ballista with a single hit. Or, you can try to lure them there, but that's questionable. Of course, ideally, casting Cold is easier, but that requires spells in the guild and a less useful Chaos spell, whereas here, in terms of leveling up, we don't lose anything.