#1 Yunalesca: First time I played through the game, I tried beating her five or six times before I decided to look up GameFAQs, which told me to get Holy with Yuna...

#2 Evare: First time I tried this, it took me about five tries before I finally killed the bastard... second time, three tries... He's tough, since you don't have the airship or anything... He'd always poison me...

#3 Seymour Flux: His cross-cleave pissed me off a bunch, as I would get so close, then he would just wipe out all three party memebers with cross-cleave

I've never found the final battle with sin very hard, not Braska's final aeon... Since, byt that point, I always had some pretty nice weapons/armour, and good aeons (magus sisters, anima, nice stats on bahamut)