This is actually an excellent idea!
1. To Zanarkand (FFX)
2.The Decisive Battle (FFVI)
3.Fight with Seymour (FFX)
4. Rose of May (FF IX)
5.Final Boss against Chaos(FF I) dont know the actual name
Honorable Mentions
6.Battle at the Big Bridge (FF V)
7.Suteki Da Ne (FFX)
8.One Winged Angel (FFVII)
9.The Promise (FFXIII)
Im sure there are more, I just cant remember now.
My (locked) thread about how difficulty should be a decision for the developers, not the gamers.
https://gamrconnect.vgchartz.com/thread.php?id=241866&page=1