My best solution to levelling is to equip as many Tech Boost abilities and EXP synthesis items you can, and just grind it out in the Gizmo Shop in Traverse Town. You get a ton of heartless in that one little room, so you can just go out a couple screens to respawn then and go at it again. This is made faster by acquiring the Encounter Plus ability from synthesising enough items, so you only need to walk out one screen before going back in.
Another great way to level up is if you're simultaneously trying to get the Wizard's Relic and Defender for Donald and Goofy, since you gotta kill a lot of Wizards and... Uhh, Defenders. For a mix of the two, the Hotel is a good place to go since only they spawn there, AND it's right outside the Gizmo Shop!