The hardest boss for me was from Ghouls and Ghosts for the Genesis. The final boss on level 5. I mean, Christ on a bicycle, he was tough. Although most of my friends couldn't get past level 2. I played through the boss at least 50 times before getting to his second form. Each time I would have to play through the whole game since I would use all of my continues to try and solve the pattern. I think I spent something like 2-3 months playing through the thing.