can be a little difficult on Proud until you get Cure. Once you have that, you should only ever die if you're careless or maybe if you get hit between cures or something.
KH 2.5 will be harder since Critical Mode and Critical Mode level 1.
Also, try playing level 1 Proud on 1.5 without using the damage storage trick then try saying it's easy. I tried to use Genie/Mushu strats on the bosses that the speedruns used them for, but I didn't know why it worked at the time, so I had just summoned them right away so it was weak.
Sephiroth is a terrible fight in KH1 anyway as it's more a level check than a skill reliant fight. if you're not high enough to have the defense to avoid getting 1 shotted or second chance, or whatever abilities make it easy. It's not really a hard fight, people just go into it unprepared, get one shotted, then complain that it's hard. It doesn't help with the 7 hits counter attack AI he has in Final Mix since now you can be forced to take hits which is one of the 2 reasons it's hard at level 1. Every other thing he does is easy until phase 3, so you should have enough items to just strike raid him or something. And Final Mix added Leaf Bracer which makes the fight even more of a joke.