The ballista is a very strong card because of:
The range you can cast it.
Its a quickcast spell
Its mana cost.
Its attack: 1-2 zones, 5 dice with 3 pierce.
Its ready marker which can be used to stack damage with other ready markers (ballistas, wizard tower etc.) AND an action marker AND a quickcast marker.
Its resilience (HP and Armor +/- other traits (flame +2)).
It starts with 1 load token.
No unique / legendary / epic
Its a complicated package to balance, and while its certainly possible to completely gimp 1 parameter its not neccesarily the best way to finetune this spell.
Unless you wish to put it at something riddicilous like 2hp and 0 armor im certain more than just resilience needs to be looked at.