Proud Mode | Never looked at an online video for help
First boss that got to me multiple times was Vanitas. In fact, every battle with him was tough. I believe I lost on each battle at least once with Ventus, but the Vanitas battles were most troublesome.
As for Terra, I believe his Final Boss was the main one that got to me multiple times. Jeez he was insane. Oh, and Zack killed me over and over. One hit-combo kills that aren't blockable usually give me trouble.
As for Aqua, the one that really got to me was the first form boss in the Final Episode, and the two secret bosses.