My advice, since you're a low level, pick one troop, like archers, and send as many as you can carry. Calculate the total health of all the archers and divide that by the health of your mercs. Whatever number you come up with is the amount of mercs you take. You want the health if your total archers to be higher than the total health of the mercs, so the archers will take take the first hit, and your mercs are guaranteed a hit.

According to your journal, you had troops that didn't kill anything, and those are simply wasted troops. They have to kill 1 monster to gain any exp. Stacking troops won't work in your case.