Some Kinda Hidden Fairy
In the second book, second level, you can turn around and jump onto a platform with 2 coins. if you keep going there is another platform with a fairy and a little health. the fairy can help you get to the mini game, and jump onto the bushes over the enemies that you can't kill.
When you playing Shrek as a human, you can bounce back the green glop that shoots out of the guns from the midgets in the factory. just press B against it, and it shoot back to the enemy.