The game is quite a challenge, but unfortunately not only because of the difficulty. Sometimes the physics is really weird. It should be repetetive, I mean that certain shots/falling objects should kill a certain robot every single time, but it is not happening. For example, the bricks on the last level are unfair <<Spoiler!>>, because after destroying the wooden plank under them they sometimes kill the robot and sometimes don't. There are many examples like this in the game.
One important thing about the last level: I got the final medal after killing the Princess. I finished this level anyway to be an honest owner of it (otherwise I would remove it from my account), but this should not happen.