The main character=Instant Game Over wouldn't be so bad if they handled it like the MegaTen games do it, by making your party leader overly more competent than the rest of the characters and giving them access to abilites that cut down on cheap deaths with good pre-planning.

As soon as I got Snow back and could make him my party leader, I noticed I died very rarely in the title afterwards; mostly from Barthandelus and the Power Clod boss fights.

I feel the game is harder only because the game gives itself some rather easy means of killing your party. I think I died the most in Chapter 5 when my party was composed of Hope and Lightning and mostly when Hope was designated as my leader. Later it was the Barthandelus fight and even then it was only because of Doom. If he didn't have that move I could have beaten him the first time we fought. Both instances are because of cheap tactics like instant death of which the game offers no absolute immunity or by placing physically weak party members as the leader.

Despite all that, I still don't think XIII is the hardest, I would really give that mantle to the original version of FFIV. Most people don't catch how the enemy A.I. works and the fact the game really does require grinding to stay alive.