Try it with a mix of slingers and archers and a couple of heavy infantry to take the hits (engage their soldiers) while the ranged units get into position (to hit the ballistas asap), use the terrain for defensive bonuses if needed, also if you're left with their soldiers against your range units move tactically to furthest range possible then fire and next turn move then fire and so on.
I'd say 5 ranged and the rest soldiers to engage their troops is a good mix, slingers are cheap enough and quick to replace
Also if you can't get to the ballistas in 2 moves you get to fire at their troops on the way, then move and hit the ballistas, best to take out one then move on to the next and the next, remember two hits take one out so get two units attacking in one turn