For this achievement you must simply pick up 10,000 zombie medals. Every time you kill a zombie they drop medals which come in a variety of gold and platinum. Zombie medals can also be found lying around stages and classmates you save will also reward you with them. These coins unlock upgrades such as new combos, music, and extra costumes for Juliet! This is cumulative so you will be always be working toward this during your first playthrough and any other subsequent ones. Nick Shake is another great method for farming medals:
If you have the skill purchased you can use Nick Shake by pressing and landing on it during Nick Roulette to have Juliet shake medals/lollipops out of Nick. Mash all face buttons except and this can help work toward the achievement. You can only use this once per Stage section.