Catapult would be fine as it is IF:
50% armor pen (makes no sense for a dude in plate armor to ignore a freaking huge rock thrown at him when it would squash him like a tin can in reality).
Do listed damage per unit in group.
This will make catapults good vs groups of units (which is the main purpose of bringing such a siege weapon since there are no walls in cities etc, nor can you bombard a city from outside to reduce defenders or population or w/e, and wouldn't even be viable since they can send a single unit to kill the said catapult on same turn you deploy), but not ideal against single units (heroes, monsters, bosses etc).
So a catapult will deal 36 dmg per unit in stack, with a 50% armor pen. If the number is too high, lower it, is simple adjustment from there. Make it 18 or 20 or 15 or 24 w/e fits best. So it does more dmg to light armored units, but still decent dmg to full plate ones.
Vote up and make it happen nau 
