or if your getting the sythesis (sp?) materials you could do a good combo of this: first equipt your Sweet Memories Keyblade (the one you get from winnie the pooh.)
then have Donald equipt the Meteor staff and Goofy equipt the Genji shield because all of these weapons have Lucky Lucky. then just whach away at any of the enemies in any world and you will start getting materials.
Saix was easy first time through.... and so were the others... but i would have to say that the most challenging one was Zaldin and Xibar. Xibar was a bit tough since he always teleported everywhere and shot you from a distance.....