By far best farm I found is near end game. When you get to the Keyblade Graveyard and fight the mobs of enemies, the battle only stops when you make the final attack via train command. What I did was summon the train and set a rock on my X button and left it there all night. When I returned in the morning I had gone up nearly 40 levels. You can also farm certain items this way too. Just bombard the battlefield by train, cancel the ride and collect. It takes a minute or so for the ride command to come back but the enemies here are weak and easy to take care of while you wait. *CAUTIONARY NOTE* 
I did this on my critical file and it honestly made the end battles way less fun because I was over leveled and blew through it like it was beginner, so if you want the level up but also want to genuinely beat critical mode I recommend creating a save file before you land on the graveyard world. 

I find that the battlegate in Olympus in the first section of the Realm of the Gods is great for exp farming. The one with all the satyr heartless or whatever they are. Granted, you have to beat the game before gaining access to it, but that battlegate earned me like 30 levels in 10 minutes. 

Yeah in terms of actually playing the game Olympus battlegate is ideal. This is handy if you don’t want to actually earn the exp. 

Is it possible to repeatedly shoot just by keeping the X button pressed? Also, don't you need to move the camera to aim at the enemies while shooting from the train?