Only boss in FF I ever had trouble beating was whatever his name was in FF9. It took me a good 10 times before I finally beat him. Although I was probably on the weak side for that boss.
I honestly don't even remember the final boss of 13. Once I beat him I was done with the game. I just remember thinking it's stupid to have a bunch of the game open up "after" you beat the game. At that point, I just lost motivation to go back and do a bunch of the open world sidequests.
13 I thought was a below avg game for the 1st 2/3, not visuals or gameplay just story and linear style, and then it became a very fun game in the final 3rd. I just wish you were able to do everything before the final boss.