if you are just talking about the story line, that is easy, if you are talking about the entire game, that might be a little harder considering sephiroth
ive never tried it, and it probably would be near impossible, but try beating sephiroth with no abilities excluding your keyblade ability and any abilities equiped by certain equipable items
I Did that with the kingdom key at level 33 with no magic, cure, items, limits, etc... Nothing but the starting combo and jumping...I dint find it too hard..... Im talking about the sephiroth one ive never tried the xemnas one... he was just too easy to be a final boss(It was a cool looking fight though)