One of the easiest and reliable areas to grind for Platinum Medals is on level 4, "Fulchi Fun Center", during the "Elevator Action" section where you have to jump across a gap using four zombies. Instead of following the on screen prompts, fail it then perform the X, Square(2) attack. You should get a sparkle kill each time, then continue to go back up to have the four zombies reappear. Fail the quick time event as many times as desired until you have the good number of Platinum Medals.