Over the course of the game, you'll see the golden and sometimes platinum zombie medals that come from slaying groups or special zombies. The smallest group you could gather would be around 3-9 medals per sparkle slaying and then higher groups or targets will give you more. This is a cumulative grouping, so don't worry about trying to get it all at once; more than anything, go through your progress to complete the game anyway. Feel free to even pick up some upgrades or abilities at the same time. The game will track it behind the scenes. Some tips: you can gather more if you slay special zombies in Soul Sparkle Hunting Mode. If you have nothing but basic zombies, Square/Square/Square/Triangle (once you've unlocked the ability) is a great zombie clearance and allows you to get several zombies at once without being in Soul Sparkle Mode.