Before explaining everything I'd like to say some general words about this this method to get more denari (coins). First of all, I'd recommend reading this tutorial when you want to know some more secrets connected with the game without finding them by yourself. It's not that hard to find different ways to become richer or defeat certain zombies, because the game's difficulty is not really that high. That's why I'd like to explain that this method doesn't have to be the best way to become rich in this game! You can try out your own ways that you find entertaining or efficient or both; everything depends on you.
Okay, so if everything that had to be told was told, let's begin!

First thing you need to do is getting proper relics to complete the set that gives you 2x coins bonus. I highly recommend doing it as fast as it's possible, because it'll be helpful later to improve your characters more efficient.
These relics are:
- Luckiest Rabbit Leg
- Duel Glove
- Nazaret Potion
You can try to collect them in levels or just buy it to make it faster. If you collected all three of them, you will see that look in your Reliquiary:
Now we can start collecting denari!

In my oppinion there are two best levels to farm and get a lot of denari: level number 20 and level number 48. I divided whole topic into two different sections to make it easier to be read.
Though, in this section I'm going to write the main differences among both levels to help you decide which one to pick.
First of all, progress in game. This game has got 50 main levels (I didn't count cowboy duels or trainings), so it's obvious that you will get faster to 20 than to 48 level. Next thing: time and amount of coins you get. Earlier level is shorter, but provides less coins; a contrario level number 48 is longer and you might get more denari.
Decision which one is better depends on you. :-)

This one's gonna be very simple. Go to the level number 20 and play.

During your gameplay don't forget to pass all requierements which were written before the action started.
If everything will go fine, then you see your results with good amount of denari! Here's one of the results of mine:

Per analogiam - you choose level number 48 and play it, trying to collect as many coins as it's possible.

Why it's described separately? Well, finishing this one needs more progress done in the game, because there are only two more levels to complete the main story (as for there are 50 levels), but it is not the main reason. This level is longer - beating it takes 600 seconds (5 minutes), however more time spent on the level is here equal to get more denari. Just take a look at one of my scores:

7330 denari, quite a lot of coins, eh? :)

I wish you all good luck with gathering denari!