This achievement is for collecting a lifetime of One Million Gold.
You can farm the redux bossfights for this. There are 2 that have good payouts and are quick:
The Mushroom fight, you can just spam Tornado Slash infront of the center mushroom and it will hit the two on the side. Though you will be popping potions a lot if you don't have the right skills unlocked, or the right weapons/runes to keep going longer.
The other one is the Homunculus fight, this is the redux of the fight that follows Holborn throwing you off the bridge. Bunch them up and do a quick Tornado slash, they should all die in a few seconds.
Each completion will get you about 3k mana.
The Mushroom fight costs 20k Gold and the Homunculus fight costs 15k Gold. You will recoup the cost plus some from the sellable trinkets. So say you go there with 100k Gold, keep going until you run out of gold then sell all the trinkets, you should at least get 100k back plus some (especially if you have the shop sale skill) so you're really just laundering gold until you get the achievement if you do this.