he should've considered that even if their strength is much lower than the enemy doesn't necessarily make them lose, war is not just a game of number sometimes even a peasant could kill professional mercenary by sheer luck though the probability is unlikely it doesn't mean it's impossible, also lower manpower doesn't mean you'll automatically lose, a good strategy/tactics could reverse the course of war/battles, since this manga world is around 1600-1800 (the age of exploration/discovery or perhaps industrial revolution, assuming from their flintlock, musket, architecture and fashion) I would say that he can use napoleon as an example for a battle tactic, and that is to defeat the enemy in details.
srry it's kinda long, I just want to share my thought :3