This thread is really long, and I don't feel like reading one all of the replies, so I'm fairly sure someone might have already said this.
Who I think one of the hardest bosses is from Lufia II. I think it's the first time you fight Gades. IE, at the not end of the game. IE, in the middle. IE, when you're on level 20. IE when you beat on level 50 and he's semi-hardish. So, yeah, he's really hard. I haven't done it. But, you don't HAVE to beat him. If you lose, the game continues. If you win, you get a sword, the next few dialogue boxes are different, and you get knocked out anyway. (Or so I hear.) (Note, dying and still continueing only works the first time you fight him. If you fight him after you've gone up the three towers, and such, you have to win, otherwise you lose. I don't want to confuse anyone.)