People who find Lou on Hard need to actually LEARN how to play. I beat Lou after a about 5 tries, but that was because I AVOIDED the solos. You use the solos to your advantage. What I did was learn the points that gave you power ups (there should be three before you reach the solo), and use them in proper manner. DO NOT DIFFICULTY UP, he'll just play as good. Good power ups are the whammy bar, broken string, and double notes. The amp overload does nothing. The whole things becomes so much easier if you aim to actually attack him proper, than to actually play the song through.
--Lou on Expert though, is a different story.
I have to agree with the first post: Mizar was an effing PRICK. I actually never did beat him, and our parents friend at the time decided to sell off our N64 games before I beat it. To this day, I pray to the Nintendo gods that JFG gets released on the VC.
Golden Sun had some hard bosses, but if you want a hard boss, try fighting Gigas from Earthbound. I played him 20+ times (and sat through the whole "My plans were to kill you all in time...yada yada yada" 20+ times). I NEVER knew you had to pray 9 TIMES in a row! The whole time I was trying to beat him in his "uncontrolled" form, I was using starstorm and special as much as I could while having jeff and paula do the healing. Then he started affecting all my characters so I randomly used "Pray" (sometimes you get that golden light that heals all characters), and instead, it did damage. So I thought, I'll keep using it!
The last most difficult boss I ever fought was any Resident Evil (0, 1, 2, 3, CV:X, and 4) boss in existence. Why? Because no one ever had enough first aid sprays! And aiming was a ***** to handle (except for 4 on the Wii, which made me wish motion control was invented sooner).