Since when is 13 hard? I imported the game back then in japanese without being able to read one thing (only hearing) and just pressed X with changing to certain paradigms when I need them. I only learned to read Haste so I could abuse X even faster. The game was made for people that dont want to think what they should pick lol
I only remember one boss in XIII to be hard, and that was Bartendalus on Chapter 11 (?).
Maybe also Odin.
The last boss in that game was not hard, but he was cheap. That fucker loved to cast Death, and you didn't get the accessory that protects you against it, you were pretty much fucked.