This is an Achievment for Kaycee's Mod and may contain SPOILERS for new players.
*** Spoiler - click to reveal ***This is achieved by beating Leshy without destroying the Moon card at the end of a challenge run.
You can do this by using the Magickal Bleach item. This will allow you to erase all the abilities from the enemies frontline cards. This item can be used to erase all the abilities on the Moon card at the end of the challenge run. Doing so will allow you to use flying cards to attack Leshy directly and defeating him without destroying the Moon card. The only other way to get Bad Moon is to pull your teeth using pliers, but you need a card that replenishes items indefinitely so you can keep doing it.
The Magickal Bleach is obtained by completing the card layout shown on the painting behind you in the cabin. Get out of your seat, turn around, and on the right-hand wall is a painting depicting a random assortment of cards. Collect those cards and place them on the playing-field during a battle to match the painting. This will reveal the item. After the battle get up and go to the painting to collect the item. There are two other items to be collected as well and doing so will grant you another achievement, Master Trinketeer.