yes, onagers work most of the time.
you could also try the testudo fromation as a roman faction, you move much slower and tire faster, but not as much missle casualties as you would just marching.
I have been told that testudo is buged, but haven't really figured out in what way it's buged...
and lot's of people play the way you play(ed) just waiting on a hill...very irritating, as a marching army you could try baiting them with HA's.

but in the end you will lose lots sof men marching towards an (archer ) army.