This is months after your battle but I'll put in a wee bit more.

Onagers work really well in forts and towns without stone walls. They don't break your stone walls (because you haven't got any) nor do they shower your archers with friendly fire.

I've found that I can have three unit cards of archers set on fire at will and use fire to be real battlefield busters.

You can control them or just let them shoot. In my present campaign, I have several forts with the onagers in them. The Germans keep laying siege to the forts. I sally and it is normal for the onagers to destroy between 1/4 and 1/3 of the besieging army. Then I march out infantry if I have large enough numbers but usually, next turn they are gone. Quite often, the enemy army flees while the onagers are raining fire and death upon them.

It took me years to figure that out. You might want to try and see if it helps you.

Strength and Honor,

Celt Centurion