How to win easy battles: Max the count of your spear men. When the charge is ready just charge your spear men. It multiplies the count of your men times 10. Its worked a lot for me but it doesn't work if the enemy charges a swordsman. Just be cautious.
The dagger is actually a very useful weapon. Its fastest compared to most weapons and it gives you bonus coins for every kill you do. If upgraded early on you'll be able to kill more enemies because they're really easy