when you get in to a fire ball throwing contest as you called it, you could sacrifice one of your cavalry units to destroy the onagers, your opponent will maybe send some units to deal with the threat. in that time you could try your full scale attack with your advancing army.
and well if that doesn't work... you should welll just try to advance your whole army as fast as you can towards the enemy.
you will lose men, but there simply is no othger way ( that I can think of ) to advance versus onagers without losing men.
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