A wall completely bestroys balista. Problem solved.
A fire spell completely wrecks Balista.
I somewhat disagree. A wall is going to be a mana sink to hold ballista off, yes, but for how long? A wall against one ballista is affective, but against two, or three?
A fireball is statistically going to come 2 damage short, then the burn token has a decent chance of taking the construct down, but it isn't definite. How many fireballs are you going to run if you're not a firewizard/warlock?
Either way, 4 ballistas (worse case scenario) are hard to argue with. That many make it hard to keep anything on the field, as a creature is cast it's shot multiple times and then it still has to deal with your creatures. If you don't one-shot the ballistas they're going to make it very hard for you to make any progress.
So yes, Ballista is definately a card that can be dealt with, but it becomes increasingly difficult with more than one on the field.
That being said, I just want more war machines so we can see this type of construct deck.