If you can slay every single soldier of an invading army while they are happily and carelessly camping, you'd be called a peerless military strategist. If you meet them on the battlefield when they are fully prepared and fighting back, you risk losses on your own side. War is about maximising enemy casualties and minimising your own (at least any war not run by Moscow). Unfortunately that's more often than not extremely difficult.

If the enemy soldiers didn't want to die, they only needed not to invade. War has a tendency to kill people. Every single soldier ever to exist acknowledges that.

Since Rimuru desires a peaceful coexistence with Falmuth, he decided to keep his military genius a secret and gave the credit to Veldora. That way the people of Falmuth would fear him less.