There's really nothing more we can say about levelling up than we have already! You're going to have to level up in Land of the Dragons or go to one of the other available worlds. Beyond that you're just going to have to learn how to deal with each of the bosses in turn. Levelling up isn't as valuable in KH2 as it is in KH1 because of the way stats work in each game. Having high stats in KH1 could break the game, but that's not the case here. It's an advantage, but it's not a game-breaking advantage.
You don't have to list them all, but do you feel you have a good set of Abilities equipped? Stats were king in KH1 but abilities are king here. They're the most important thing in your arsenal. Are you using as much AP as possible? I mention because you said that you didn't have the Explosion ability... but I realized a few days later that you should have it by now. In Vanilla KH2, it's unlocked in the first trip Agrabah. Equipping Explosion will make things much easier, it's very powerful. There's a reason it was moved in the remake, it practically breaks the game all on its own. Leaf Bracer should also be equipped, and Once More if you took the Rod or Shield (Sword gets it later).
Given that you keep having trouble despite being over-levelled, I worry there may be more fundamental trouble that we're overlooking. If your abilities are up to date, how is your armour and accessories? Do you have Cure loaded into your quick-spells? It seems you didn't know how to even use quick-spells a few posts ago, but they're really essential to winning the game. If you aren't casting Cure via quick-spells you're putting yourself at considerable risk. Is Donald with you, also casting Cure? (Don't put Mulan in his place.) Have you considered bringing items with you? How are you doing at dodging attacks?
I just reminded myself: you can put lightning resistant equipment into your accessories to help with the Storm Rider. The only other advice I can think of is to summon Stitch. Stitch will recover your MP, meaning you'll almost always have enough MP to cast Cure while he's out. Sadly the other Summons and Drives aren't quite as helpful with all this moving around.
The best way to grind in the entire game is during the second trip to the Pride Lands. We can tell you the trick once you get there, but first you have to fight the Cursed Pirates in Port Royal again. If you're willing to risk that, go ahead.