I am of course talking about the international version of FF10 rather than the easier US version.
The US version didn't have the Dark Aeons or Penance.
I never actually did get to fight Ozma in FF9 but I can't imagine it being tougher than the Dark Aeons.
For a quick comparison:
FF1-4: were there any particularly tough optional battle?
FF5: Omega, probably tougher than Omega Weapon in FF8 but beatable
FF6: Pretty much all of the World of Ruin was optional, but no particularly tough enemies.
FF7: Ruby and Emerald, when it came down to it these guys were easy since it didn't take long to do any required preparation.
FF8: Ultima Weapon wasn't hard at all, if you can actually get down there without dying then you should have no difficulties with the fight itself. Omega Weapon is somewhat challenging thanks to Terra Break but the defend ability comes in handy here, not to mention the prize for beating the Card Club.
FF9: Heard nasty things about Ozma
FF10: The US version has the custom monster creations, all of which are far tougher than anything before FF9. Especially the last one, Nemesis I think. I haven't beaten these yet because I haven't finished getting all the items I need. Then there's the Dark Aeons and Penance in the international versions which ALL have attacks that hit everyone for a guaranteed 99999 (unless you've got some serious protection).
Detty. Professional Expert.
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