After purchasing the third shop upgrade and unlocking the desert dungeon, you can unlock the banker on the town board, which will make him appear in the town square during the day. By talking to him, you can invest any amount of gold. Returning to him on any day later, he will show you a percentage of either profit or loss on your investment. You have the option to cash out your investment and receive your money back; doing so and taking a profit five times will unlock this achievement.
After cashing out, talk to the banker again and he'll tell you to return in a number of days. Note that you'll have to talk to him exactly on the day he tells you, or you'll have to wait another number of days as he's busy "helping other clients".
This is very simple, just requires waiting many days. If you want to grind this out quickly, simply sleep repeatedly until the required day.