Far too easy. I played with the Necromancer and just attacked with my strongest attack repeatedly. I never defended, I never used in game items (I increased my vitality once and my magic once and that's it), and I never even came close to losing a match.
Showing video ads between levels is also a bad point. Players should be rewarded for finishing levels, not made to sit through commercials. And when I beat the game, I got a "You Won" screen and that's it. Very little sense of accomplishment.