Get the Final Form which is in the Under World. Once you have it go to the World That Never Was and fight the Nobodies. There is a lot of them so you will level up fast.
This requires the gullwing keyblade, the ability
:experience boost and for you to have unlocked the world that never was. First, go to the room before where you faced zigbar. (You don't have to, but I prefer there). Next, equip the gullwing keyblade and the ability, experience boost, if not already on. Also, take off all the abilities of the partners you choose to fight with. I never use goofy with this because his sheild can get in the way by attacking enemies or by blocking their attacks. Enter the room and let them drain sora's health. If you are playing on critical, like me, let the health bar go down about half. If you are playing on any other difficulty, then allow sora's health to go down a little past the crown pendant he wearing. From here, whenever the enemies attack, use reflega. You could also kill them directly, or use thundaga, but it leaves you wide open. That's about it.
P.S. Heal often
P.S.S. Don't die!!!
For this cheat you need to have completed Agrabah the first time, have the Oathkeeper Keyblade and have valor form (duh!). The first step is to go to the Treasure Room inside of the Cave of Wonders. Next equip The Oathkeeper Keyblade and a really weak Keyblade. To make sure your friends don't steal your experience put them on Sora Attack. Inside of the treasure room there should be a Giant Mushroom. Activate the little game. Shortly after the game has started turn into valor form. Attack the mushroom as much as you can without using finishing moves. You can use finishing moves if you don't have that many attacks in your combos but otherwise, do not. If you do it will take up time, preventing you from getting as many hits in as possible. When your drive is getting low it is time to exit the room. After you are out of the room go to the save point and leave the world. Now go back and repeat the whole process over again.
Note 1: This does work better if your Valor level is higher.
Note 2: This may not work so well if you are a very high level because the point of the little game is to defeat the mushroom.
Note 3: Using a couple finishing moves may benefit you but do not do too many.
Note 4: When I did this the maximum experience I gained in one round was 184. This process should not take any longer than a couple of hours to completely level up the form.