Just a bit puzzled on this one: "Down at the earthworks another volley dropped more savage onrushers fell to the waves of focused rifle fire. "
More importantly, the sentence is problematic because it is two sentences glued into one. A comma-less comma splice.
I think 'onrushers' even though not an actual word, are about the only clear thing in this sentence. You should really try to re-write all of it more clearly, and after the sentence is clear and precise, you can play with invented words if they fit.
I agree with Jazz. Though onrushers may not be a real word, it clearly answers to the same logic as onlookers, which is a real word. Had the question you asked been about a different part of this sentence, I would not have given onrushers a second glance. It makes perfect sense to me. I might have looked it up later, discovered it was not a word, but thought to myself good use of creative license. The sentence structure on the other hand...
ditto that ditto! per the first 2 pages of a google search, we're not alone in liking that coinage, as the op isn't alone in using it...
what does that have to do with using the word 'onrushing,' or gluing two sentences together into one that makes no sense?
No, that dynamic is fine. That's not what we're talking about. This sentence is grammatically incorrect in that you have glued two sentences together. Frankly it looks like the kind of thing you see in phone texting. It should be: Down at the earthworks another volley dropped. More savage onrushers fell to the waves of focused rifle fire.
Ahhh. I see. Sorry, couldn't see the woods for the trees there. Here's my fix: Down at the earthworks another volley was unleashed, causing more savage onrushers to fall to the waves of rifle fire.