I find that Guizmar (don't know the english name... Guisamaluk ?) is rather hard. And Beate too (first battle).

Ozma isn't so hard. The first time, I had beated it at lv.99 (I was a noob), but second time at lv.40 without any problem. Ans it's possible to beat it at lv.1 (there are some videos on Youtube..).

well...I'm a noob since I've just started playing this game.....and the hardest boss so far was The Master in the Evil Forest(if I got the name wrong then it's that plant which holds Garnet). I got like two times the Game Over screen....damn his thunder!

Hmm. I'd say Necron. When I was first time playing this game (it was the first FF game I beat), the last boss was incredibly hard for some reason. I didn't really care about the char development system and didn't bother leveling up either. Now when I think about it, I wonder how I ever managed to beat him with those stats, lol. Yeah, and Grand Cross was a pain in the ass. With some really bad luck you could get all your characters killed at once!

Also, Ark was pretty hard with Zidane, Vivi and Dagger. xD

There are some hardcore gamers out there, defeating every boss at level 1. To be honest with you, I'm not one of them.

My hardest would be beatrix. I just wasn't prepared for climhazzard, shock and thunder slash, which she did all in a row.

necrons grand cross can be easily avoided if you ribbon your characters(equip all immune abilitiey like jelly,antibody,locomotion etc)

id have to say ark it took me forever to beat it with steiner,vivi,garnet and zidane