Greed is a merchant commonly found throughout the realms. He sells 3 items for blood droplets, a currency you accumulate by killing enemies. All you have to do is buy all 3 items in a single encounter-- the achievement is NOT to purchase every single item Greed could possibly sell.
Depending on your price luck, it could take several thousand blood to buy Greed out. If you're struggling, I recommend saving up until you've seen Greed a few times during a run. There is also a helpful skill called "starting package" that lets you begin a run with 500 blood.