The balance on this feels terrible because of the fact that damage is tied to the result of the math problems. Especially with the subtraction monsters, when the answer is 0-2 you've solved a problem for no gain and just wasted precious time. The monsters move REALLY fast, and you just cannot ever do enough damage.
EDIT: The monsters get frustratingly difficult as you move on. The icon monsters can't even be solved until they are halfway to your castle, and it's no longer arithmetic, it's counting. The division monsters have the same problem as the subtraction monsters, except that they fly over the slow trap. I am finding myself spamming the Right Arrow key hoping for a problem with a double digit answer to be able to keep up.