You must spend a total of 10,000 medals at the Chop2Shop.Zom for this achievement. You will likely get this while purchasing items to work toward other achievements such as upgrading Juliet, BGMs, combos, and other various items. The key to getting lots of medals is Sparkle Hunting kills so buy the Holy Chainsaw combo and use it on crowds of zombies. Stages 3 and 5 both have a wave of 32 zombies toward the end of them that open opportunities for lots of medals. Those are good stages to get lots of medals in with this combo. Use the medals for purchase at the Chop2Shop.Zom to help work toward this.
See Love Nick for medal grinding methods.