Where's the best place to grind before continuing in Neverland?

As a matter of fact I recently finished Neverland at level 39-41ish myself. It was not significantly difficult. My tactic is basically spamming the cure/cura (you recieve the upgrade right before Hook) spell. It's kind of cheap and desperate but it works. Then when you get a few strikes in on the target or surrounding heartless your MP is replenished. But it's also helpful to have elixirs and potions on you.

With Hook just stay air born as much as you can, you'll receive the flying ability from Peter Pan beforehand. Hook hardly ascends himself but when he does that's when he's most vulnerable but he's also fast as hell. I assume you gotta get a good hold of your own flying to be able to strike at him that way. I failed and had to rely heavily on my cura tactic.
Also I read, though too late to try it regrettably that fire (at this point it should be fira or you're right, you're are definitely there too soon) has a pretty powerful effect on him. It will send him running around in a panic with his pants on fire which does him gradual lingering damage for a short time.

 

I have always believed that Traverse Town is the best place to grind levels. I used a path where I got from level 50 to 99 in 3-4 hours, which is really fast to gain all those levels. Here is the path:

Third District > Second District > Hotel > Gizmo Shop > Roof > Roof entrance back to third district

This is the basic route, just try and hit the spots where the enemies show. If sniperwild shows up in the second district, just leave the area and come back. Good luck!