My hardest boss to date is Lavos 2, solely because I think my game has a freaked-out power stat for him. Almost every time he used Grand Stone, my party was instantly 90% dead. That meant 2 people dead and one barely alive. That was at full health -- otherwise, instant KO. I wanted to reach into the screen and kill him myself after a while.
Magus was just time-consuming, not hard, to me. I kicked his butt first time around either way; the barriers made it longer.
Giga Gaia was a challenge, but not so annoying that I wanted to throw my controller. That's about my standard for boss super-difficulty.
Golem Boss was easy, even keeping him from running away, to me. I died to the Golems I was supposed to die to, and since that's part of the game, I don't consider them real boss battles. I know you can beat them, but I never bothered to level up that much.
I also never bothered to fight Spekkio in Nu form, so he's not on my list at all; by the time he got there, I was weary of boss fights and just wanted to get endings. Spekkio didn't matter that much to the game in terms of his battles, so I ignored him. Maybe I can count him as the hardest, because each time I fought him before, he creamed me, but I think I was under-level, so I won't count it.
I also won't count Flea -- she was the first boss I ever fought because my friend plunked me down into the middle of the game and I had no clue what I was doing. I was so happy when I finally got to Slash...and then I died. But the battle was so memorable
Whenever I hear the generic boss music, I think of that one.
Optional bosses were no big deal, except it took me a fight or two to figure out Son of Sun. He was a pain, but not super-hard once I figured him out. (Area attacks = bad.)
Masamune was in the middle. He took a while to die, but wasn't much of a problem.
Some people mentioned Queen Zeal. Hallation made me want to scream, but I had so many Full Elixirs (Is that what they're called? I forget. Those super-healing items.) by then because I'm item-paranoid. I never use anything powerful or super-healing, because I know that someday I'll hit that evil boss battle where I'll burn up my entire inventory...most of the time I end up keeping them for the whole game because I always expect someone worse...
Come to think of it, I was fairly well-prepared for every boss battle except Lavos' second form. I may be forgetting a boss, but I don't think so.
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