I am terribly embarassed, but the second boss is practicly impossible for anyone with bones in their thumbs in stead of rubber:
TaleSpin(yeah, the Disney one) for GB. I never finished it. As a kid I never could, as an adult I realise it's practilcaly impossible. I have only beaten the second(!) boss once, by doing, well, something, I don't really know, but I never even damaged him since.
Let me give you a description:
A big robot thingy, with a small cockpit with a dog. In front of the robot float 2 iron balls that damage you on touch, but home in on you horizontaly, then drop down. On top of that, it shoots balls.
Trick is to get in front of the balls, but the dog sits in a sort of U shaped cockpit, so shooting straight is no option, bounces off, and to get too close means death by dropping steel from above. I HATE the game.