If ever I use them, it's with flaming ammo and deployed just behind my infantry so they can't hit them (they never really hit 2m in front of them). When the enemy are close, I stop them firing and charge with infantry, flank with cavalry as usual. However, you should ALWAYS keep at least 1 cavalry unit behind to protect them - the enemy cavalry loves charging in and massacring your precious artillerymen (as I'd do to them).

But generally, I can usually never be bothered in campaign to drag onagers for 10 years from the one city which can build them to the front, only to have them destroy each other or get mowed down by infantry - I mainly use siege towers for big walls, rams for wooden ones and infantry/cavalry for pitched battles.

I think onagers are best for "joke" custom battles - e.g. 1 stack of onagers firing across a river at the enemy whilst they try to rush across the bridge and meet them.