If you are having a hard time leveling up when you get to around level 50, then follow this cheat:
Once you obtain the 'magnega' magic skill, go to the pride lands for the final time (you'll have to unlock a key at another world, I think Atlantis, to get in). After defeating the huge heartless, go to the King's Den. Then, go up to the Peak. Set Donald and Goofy to Sora Attack. Then go to the center and use magnega. If you set your weapon to Sweet Memories and have Lucky Lucky on for all party members, you will get tons of EXP, lightning shards and magic orbs! Go to the King's Den and back and repeat the process. Enjoy!