I've played only two imperial campaign so far, before and after patch 1.2. Both on VH and with the Scipii. I based all my tactics on Onagers, giving a battery of 3 as soon as possible to every army and making a sort of siege park in reserve when I start to assault cities. They did wonders, in attacking batlles, cause they burned from 60 to 150 men each ( I'm playing with 160 men's unit). Using flamed loads I usually target the central unit in front of the enemy general's squadron. Rounds spreaded as you know, but you have a good chance to flame the general ( I experienced a 25% ratio at worst). If the enemy had some tough nut, like elephants or cataphracts, I switch one of the onagers, the best ranked to precision fire with solid shots to soften'em, if the elephants shows no willing to advance fast I try also the flamed shots to "ignite" a stampede.
The best onager's scenario, a true heaven, is the bridge defense. Here also the more linear deployment of the 1.2 patch is forced to switch in long columns. You must only target a mid column unit and watch for the slaughter. You must only be quick to switch target if the unit start a charge to avoid the back shot on your defenders. If you think that the pause command is too gamey switch to solid shot, they seldom to never cause friendly fire when you hold your position.
Also in pure defensive battles onager shines. Yes enemy rushes, but a couple of well aimed shots are possible and if the enemies had tight formation like hoplites or phalanx, every hit did 20 casualties. then you could turn you aim to one of the flank. With solid shots you can slow or stop the usual cavalry flanking.