I have to agree with Ryastar, the AI is not the smartest of things but it can be very efficient at times. As your progress in the game, more and more enemy heroes will use AoE damage spells like geyser, fire rain or lightning. And the AI will usually target the stack where it can do the most damage, of course some stacks take priority, like (royal) thorns, the AI loves bashing and burning those. Besides, you don't get to complain about difficulty or brag about scores if you're a mage, go play a warrior, preferably on impossible, and then you can whine.
