I'm never one to discourage people from buying dvmatte :) but I don't think they used very much greenscreen in that video. For those wide beach shots, the greenscreen would have had to have been far too big. So they probably used the inherent contrast of the soldiers (dark) against the beach (light) to create simple mattes for the soldiers. Since the background is relatively static and the figures are so small, it works fine. It wouldn't work if the background was more complex (for example, a city street), or if the figures were larger in the frame (you'd see holes, and wind up rotoscoping them).

To replicate this on dark dirt, you'd want your actors to have very bright clothing. :) Which may not work out so well for a battle scene. But it would be worth a try!

The other approach would be to divide the screen into 4 or 5 sections from left to right, and have your soldiers act out the scene within those divisions. Then all you'd have to do would be to make a rough box around the action in each of the clips, and layer them together. That would at least get you 4 or 5 times the number of actors you have, and you wouldn't have to do any complex extractions...