You can get this achievement legitimately by having a ruler with 15,000 in prestige. If you get powerful enough (more than one emperor title) and win wars and such you will eventually get this naturally. There is also an exploit using the different patches.
Right-click on the game in Steam and go to Properties -> Beta and choose any version before 2.3. Start up the game in ironman and save it. Now change back to any version after 2.3 and all the numerical values will be ridiculous numbers (+2124109351 gold, -127418250 piety, etc). Unpause the game and let it run for a bit and you will get the achievement as the numbers will fluctuate wildly. You can also use this method to get the related achievements for piety and gold.