A few ideas I had that can make a siege mage a viable option anywhere were these:
1. Shrink object: It's a low level spell that allows anyone to easily carry a ballista around with them, then throw it on the ground and activate your turret.
2. Stone/force/ice/whatever wall: Since he can link to the ballista, he can turtle up. surrounding himself in walls so he can't be hurt while his crew loads the weapon.
3. quicken spells: Since your focus is on the ballista, you won't be using as many attack spells. Quickening true strike basically means that the ballista is NOT missing.
4. Vital Strike: A Baliista doesn't get precision damage like sneak attack, but there's nothing in the rules that says you can't use vital strike. Meaning a light ballista can deal 6d8 damage, along with the bonuses from sacrificing spells.
Some thoughts I had :)
