This will most likely be your last unachievement - unless, like me, you grind this before facing the final boss. By this point, if you've been collecting most of the doubloons during the game (and have smashed most of the big vases), you will have about 500 - 700 thousand.
After spending sometime grinding, I have discovered, what I think to be, the quickest way to make money. Doing this will bank you 100,000 doubloons in approx 25-30 mins.
I recommend doing this achievement last - after obtaining all of the power ups, life/special upgrades. It's probably also best to do the Mask of Gods achievement before this; your sword becomes twice as powerful when you have collected them all.
OK - I found the best level to do this on is the secret area: 'Jungle - On the Treetops.' To get to this level you need to break the ground using the smash move at the bottom of the map in Jungle Highlands. This level has one teleport at the left hand side and one exit at the far right. Initially, I started running from left to right/teleport to exit and then back again, but found this a little slow. Then I discovered a quicker system which can be done in 2-3 mins.
You are going to start a 'cycle' using the right side of the map.
Now here's my most important tip - use the uppercut move all the time (well, on normal enemies and small spiders). This move gives you twice as many doubloons per kill. With the upgraded sword, all small enemies require two quick uppercuts. Another benefit of doing this move is that the coins are launched into the air, allowing
you to collect them more easily.
First, start at the right hand side of the map. Work you way down to the bottom right (there's a big spider and a small spider down there). Now go up, all the way to the top, killing everything along the way. Use the blue platform to get to the highest level and use the beam attack to destroy the enemy which rushes at you. The beam move is also useful for killing the big spiders and the big guards with swords and shields (note: jump over these when they pound the ground and hit them from behind). When you have killed the big sword guy on the highest level, you will come to some ladders. Go down the ladders and kill the small sword guy at the bottom. NOW, head back to where you started. KIll the guys on the platforms and there is a big sword guy worth killing on the bottom level as you go. Once you have killed everything here, rush back and refresh the level. This process should net you 8,000 - 10,000 per run. Expect to get 2-3 giant doubloons each time (worth a thousand each).
Expect to spend approx 2 hours grinding using this method.
Hope this helps.