Depends on what you mean by hard.

Just from seeing the bit where Ultimecia randomly decide who is in your party, in spite of the game making it seem pointless to keep magic junctioned to people not in your active party? Well certainly you will get your ass kicked, but that's just cheap.

FF XIII's Orphan having that insta kill move? Just straight out cheap. Protect from that and the fight is a cake walk.

Not beaten FF III so no Idea about Cloud of Darkness, but Zeroumous from IV always gave me the hardest fight.