I have yet to beat the original PS2 version of Kingdom Hearts II - I got stuck when I was a kid on the morale missions in Mulan's world. They're basically timed missions, which I had problems at the time, such as the flooding section in Ratchet & Clank or the parts in Prince of Persia: The Two Thrones (the Dark Prince gameplay) and Devil May Cry 2 (some area in the game I don't exactly remember) where your health drains gradually overtime and kill you if you don't beat them fast enough.
I had beaten Kingdom Hearts II Final Mix on Proud Mode on PS3 in 2016 or so when I got the compilation games on that system, after beating Kingdom Hearts Final Mix on Proud Mode. I had trouble with the World of Chaos boss in the latter game as Sora kept taking so much damage from the laser attacks.
I recently beat those games again on PS5 picking up a magic based build for both of them, and Kingdom Hearts II on Critical Mode for the first time. I think using the synthesis system kills the difficulty of Critical Mode... I had late game equipment that boosted attack and defense a lot thanks to the Lucky Lucky abilities granting a lot of synthesis items.