Cult of the Lamb

This achievement is NOT cumulative. You must have at least 666 gold coins in your possession to unlock it.

This is going to seem like a huge amount, especially early on when you're spending most of the money you're earning, trying to keep your Followers happy, fed, and healthy. However, once you get your camp set up with the necessities, such as beds, food, and such, you'll find you don't really need to spend money at your camp anymore, or at least not very often, and can therefore amass money quite easily. Specifically, as you run through each section attempting to beat each boss without taking damage, you'll assuredly require multiple attempts on each boss. Each "failed" attempt will still result if you killing the boss and bringing back 50-100 coins per run, with each run taking under five minutes. Before you know if you'll far surpass 666 coins while working on those four achievements.

So that is the fastest way to amass money in this game: replay an area where you've already beaten the boss, finish the starting stage and then immediately choose a combat stage, break the idol statue, take the teleporter to the boss, and quickly beat it. Collect the money, teleport back, and repeat. On top of that, assuming you've gotten the chest in your camp that allows you to sell items, you could potentially build up many items that you just never use (such as certain seeds or food or whatever) and sell off dozens or even hundreds that you've amassed in your runs to quickly generate some money just to hit this goal for the achievement.