How I done it was set the drop rate to the max using the cheat, then starting at Traverse Town (don't remember if DI has any heartless in questploration, if it does start there) and just collect the loot. Once you have the loot, change the difficulty and grab loot again. Once you have the loot from every enemy for all difficulties in Traverse Town move on to Wonderland and do the same thing.
http://s7.zetaboards.com/Burnin_Rubber/index/ Fan board for most racing games. Just updated for ease of use. Enjoy.