This achievement is best found in the new Endless Mode, but can also be done in the Moon with how long the mission is.
Load your deck with as many gold gain encounters (The Maiden, Goblin Town, Alchemist, Arm Wrestling, Knight's Sword) but try to get yourself the equipment that rewards you food/gold, especially for combat (Bounty Hunter Helmet, Gambler's Mask, Hunter's Blades, Gladiator's Gaze, Golden Sword/Golden Shield.) You should always switch off wearing either the Gambler's Mask if you can reliably keep yourself at full health, or the Bounty Hunter Helm for guaranteed gold after a fight.
Whenever you get the Golden Sword or Shield, sell it immediately for 50 gold, so that you have a chance of getting rewarded with it again on equipment gain. Other than that, make sure you're packed with enough equipment gain and food gain so that you don't have to buy things as you go, and you can start hoarding gold.