What level should i be when going to the worlds?

Really in Kingdom Hearts1 as long as you kill every heartless you come across in the each world you should level up decently. There will be some areas where the heartless wont stop coming. The little shadow heartless only give you 1exp so there is really no reason to focus on them. But when you get to levels like never land hollow bastion and end of the world you may have to go back to some previous worlds and just kill heartless for levels. Cause those are the worlds where it gets pretty intense. If you want to really make the game easy from the very beginning, just go through the first world then go back to it before moving on and go to every area and kill the heartless. Then as you clear the worlds go back to each of the ones you've cleared and do that. As you progress in the game the more worlds you will have to go through which means extra exp. This tactic works i have tested it. Its the only reason i have made it so far in Birth by Sleep critical mode.