Overall: Use Supporter that will boost your attack or increase your GP. For Rookie and East Blue Difficulty, the supporters will barely have time to Load. So you won't be using them much, therefore you need them to boost your stats at the start of the fight. Best item is the spice, because it takes away the opponent's ability to block. Memorize the mushrooms, one of them gives you a stat boost, one of them can poison the opponent.

Little East Blue: I have noticed that the fastest way to hurt the opponent, at least on 1vs1, is to run all the way to the right. They will usually be airborn, they will fall (so they can't block), it's easy to sneak an special move.

Sabaody: The opponents will attempt to jump to the left, to seek for items. You can dash to the right and get in their way, so that they never run around the level aimlessly. Don't stop them at the very right of the Stage, preferably stop them in the middle, because the rightern-most side has an elevated floor (therefore the opponents won't have time to jump after knocked to the ground, and they will lay on the ground fainted for a while).

Alabasta: I think the AI has 4 main modes: Object seeking mode, Track and fight mode, Block and protect mode, and Jumpy mode. If the AI is jumpy and the character has a diagonal air attack, you can wait for them at the corner of the ice patch. The AI will come and attack you on Hard Superhard.

Impel Down: I think to sneak in a Special Move it's good to climb to the ice layer, then fall down to the lower floor. Trigger the AI to pursuit you. While the AI is airborn, they cannot block your Special Move.

Marineford: If the opponent does not exit the Ice zone, climb upwards or corner them. They cannot slide away after hitting them when the floor is short, they will enter the freefall state and you can keep hitting them. The lower floor is too abnoxious unless the opponent is cornered.