Oh. Do your favorite worlds first. It really doesnt matter. I recommend doing it in order of KH, but like i said, it makes no difference.

Hit every barrel u can to get MP (moogle Points or Synthesis points or w/e they're called) to get alot of good cards. The first tim I went thru i got like seven Cloud cards that way by like the fifth world. XD

Lvel up, which is really easy in Riku's mode and semi-easy in Sora's. Party cards are USELESS unless used i sleights (one of my fav sleights if 'Flaming Donald' did anyone else get that one?)

Save frequently and hold down start (or select I foget which) to skip thru cutscenes. And LEVEL UP. this makes the game oh so much easier.