Thanks so much for this. You were right: soldiers had no real answer to beetle shells, and the game never explained it. New version is out with a few changes based on your comment:
Bigger armies now matter: shells can't block as much of each bite anymore. When beetles shrug off your soldiers, you'll see it (metal sparks) and the game points you to the counters: the Stinger upgrade and bullet ants, whose poison ignores armor.
Feedback like this really shapes the game, thank you!