To unlock this specific achievement, you will need to have cumulatively earned 1e40 Gold across your adventures. If you are not using Scientific Notation, this will be about 10.0D - Big "D", for Duodecillion. You can somewhat check your progress in-game by going to the "Milestones" tab/menu, going to the "In-Progress" view, then looking at the gold milestone under "Clicks, Kills & Gold". When in range of this achievement, you should be working towards "Impressive Stash" for 99.9T (Tredecillion) gold. Lastly, it may or may not unlock while you are playing - I had to restart the game and load up before it went from 99% done to Achievement Unlocked.
Before going further, please note that Idle Champions is a game of multipliers upon multipliers, and gold-gaining is one of those many multipliers. When starting fresh, the monsters killed will give you a single gold piece each at area 1. This number increases based on various factors:
1) The area you currently are in
2) The amount of unspent Divine Favor you have banked for the god of a region
3) Any bonus multipliers by champions currently in your formation, their skills, and equipment
4) Any Blessings you have unlocked that affect gold drops
5) Any potions you have active
#1 and #2 are the largest factors in getting to the point you can even get near the level of gold required for the achievements. #3 is somewhat useful, especially earlier on, but eventually you'll want to focus on damage boosts to let you get to a farther area, which could outpace whatever +gold % you would have earned on an earlier area. Also, the "Grand Tour" area only gives you space for 9 champions, which limits any multipliers for added damage potential. With "Tomb of Annihilation" or "Waterdeep", you get 10 spots which allows additional multipliers to boost your DPS champion.
You will eventually hit a 'wall' where the enemies' HP jumps up by 2x, then 4x, then massively more than you can output. It takes longer to buy enough levels on a hero to unlock the next upgrade to manage against their increased life, preventing you from getting to higher areas and slowing down your kill rate (which means less gold earned). You will then need to do a reset by
"Completing Your Adventure", which grants you banked Divine Favor. 1 Favor is +1%, so 100 favor will double your base gold, and 900 favor will increase your gold by a factor of 10x.
While resetting may seem painful, that Divine Favor is important, so get used to it. That 10x factor means you get a lot more gold early on, unlocking champions earlier and unlocking their upgrades sooner. If you can now afford 3 new upgrades, each giving your group +100% damage, you will actually do 8x the damage, which can let you get just a bit further to a higher area, earn gold more swiftly (a factor of 10x in this example), and earn more Divine Favor. Keep repeating, and you can push to higher amounts.
Additionally, whenever a Event or a Time Gate ends, you will be forced to convert the Divine Favor of those alternative Gods into Favor for the main areas. This favor is converted to a percentage of your current Favor for a main god - for example, if you had 1B Torm favor versus 100M Kelemvor favor, and you converted a good Event run, you could choose 800M Torm or 80M Kelemvor. If you did Kelemvor runs before event closure and got it up to 500M Kelemvor, the event would then convert to 400M Kelemvor favor, a gain of +320M on top of the 400M you worked for.
As a general example of where you may be to earn this achievement: After on-and-off grinding Events and unlocking characters, I worked on the "Tomb of Annihilation" quest lines and earned about 5 billion "Kelemvor" favor. Under the first Tier of Blessings for Kelemvor is "Golden Riches", which offers +200% goldfind. I ended up using Paultin (Event L4) in place of Jarlaxle because I was also using Strix (Event L11) and Nerys (Event L12) to help boost Strix as my core DPS (and then boost Nerys' Spiritual Weapon). At this point, I was able to reach around Area 265-270, and I was earning about 1U to 6U a kill. After letting it play for a while at this area, I slowly was earning % to the 10.0D I needed, so I just let it run and farmed the gold for this achievement. Enough that I got about 50B Divine Favor, around 10x what I previously had banked, once I finally reset my run. You may want more Favor - at 50B, Area 265 could be giving about 10U instead of just 1U, or you could farm at a lower area 260 which might be around 1U at that point.
Note that, as of around April 2019, there was a big balancing patch (The Base 12) that acted as a nerf for a few characters (Tyril especially) while becoming a major boost for others (Jarlaxle becomes both DPS and a greater gold-booster). This has changed the 'Meta' of the game as to which characters are 'OP' or not. This also means that there may be changes later, particularly for secondary characters (from events and from missions) which could also effect your general gameplay, whether for this achievement or for another.