For the important action involving the main characters I presume it would be storyboarded as normal. I'd guess for the background shots showing the other soldiers fighting without the main characters being involved, you'd get a lot of second unit shots and pick the best in the edit.

Well, annoyingly nearly every character in this shot is as important as each other. It's a gunfight where a band of rebels are fighting an army at the start of the film only they have set traps for them such as spear pits and log drops.

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You can't keep 'em out, they're already in!