First kills for monster is always hard... and once you killed lvl 14, there will be lvl15 waiting for you
my general feeling
* look at how many casualties you have, fewer of your troops killed means you did better
* send equal amount of each type of unit (infantery, cavalry, artillery and bowmen)
* some lvl is better than others, for example lvl5 artillery is better than lvl6, so play with these numbers
* of course march size matters, so every time you upgrade your drillgrounds, try again that monster that you could not hit before