GUNZO!

It never mentions how to beat the game while playing so here you go:

You have to kill every enemy in the game to beat the game. Killing the last boss won't be enough alone. Also don't get fooled by the ammo boxes around, you have unlimited ammo so you don't have to think much about efficient usage of bullets.

Every mob has a specific range before they start to attack you. Don't get too close and spam your bullets until you kill one once you are in range (you can not shoot them from afar). Reload before approaching another one.