Playing this game, I realized that preparing for a boss encounter was tantamount. Some of the baddies in this Final Fantasy were ridiculously tricky or just plain overpowered. The battle with the four crystal-like things that have different elemental properties comes to mind first for me. That freaking fight got on my nerves so badly. (It's been a looong time since I played it so I don't remember the name of the boss or the context of the encounter.)
Which boss battle was the most taxing for you to get through when you played this game?