Mini Bosses: Behemoth
You'll have to fight 3 Behemoth along the way to Bahamut. You'll die for sure if you don't know the trick. Behemoth Counter attack when you hit him with physical attack and Counter a nasty magic attack if you hit him with magic. The best strategy to beat him is to use Blink on Cecil and Edge. Cecil attack normally, Edge throws Shuriken, and Rydia summon Titan. 

Boss: Bahamut
Try to kill him before he uses Megaflare.