You will most likely get this achievement during your second playthrough. It is cumulative, so all of the gold you earn or loot in your first playthrough will carry over to your second. Also, you don't need 10,000,000 at one time. The game counts all the gold you've ever held. To see the total gold you've earned, go to your History from the menu and find 'Total Earned' in General under the Adventure Log tab.
One way to obtain lots of gold is to defeat bosses in the Everfall post-game. You will receive gold and other rewards which can be sold. Your best option is to defeat the Ur-Dragon through the port crystal in Cassardis during your second playthrough. Sometimes he will spawn with low health and you can get an easy set of weapons and armour to sell. Also, your weapons and armour at the time of his defeat will become dragon forged and can be sold for large amounts of gold.
Another option is the carrot exploit. Put everything you own in storage, then go to The Rift and find sorcerer Pawns with the augment 'Sausion' (this part is unnecessary, but makes this go by faster). Holding the Badge of Merit (given to you by the Duke during "Reward and Responsibility" and an Idol if you have one, go to Lola's shop in Cassardis and buy as many carrots as you can for you and your pawns. Now sleep at the inn in Cassardis 3 times to turn the carrots into moldy carrots. If you go to your options and change to 'offline mode' you won't have to wait for the game to access the servers every time you wake. Go to Aestella's shop and sell the carrots for around a 250K profit.
As soon as your 'Total Earned' reaches 10,000,000 G, the achievement will unlock.