What is your go-to army composition?

[Sp4]

As to ballista: arty is definitely an aggression trigger. Any arty. The only time it is not a trigger in field battles is if the AI has similar arty to match.

No.

[Slaists]

No.

Do a quicksave before any battle where you are attacking and you have a ballista (one is sufficient) whereas the defending AI has none. Does the AI defend or attack you? OK, exit the battle, load from the quicksave, delete the ballista and attack that same AI again. Unless you have missile superiority of other type, the AI will defend. I am talking about a field battle here, not a wall-less town defense.