This achievement is extremely luck-dependent and will probably take several characters' lifetimes to get. To get this you must first get the Administrator lifestyle trait. To get Administrator you must pick the Rulership focus. Every so often (I think usually 2-4 times a year) you will get a popup with some decision to make usually between two fighting courtiers/vassals. Pick whichever one you want and you may get a random positive event which will sometimes "level up" your rulership which you can see as a permanent modifier giving +1 or +2 to Stewardship. After three times you will get the opportunity to pick the Administrator trait. There are lot of other random events thrown in there and most of the time you don't get any bonus event at all so it often takes decades to level up all the way. Throw in Rulership almost certainly giving you the Stressed and/or Depressed traits and your ruler will often die in their 40s or 50s unless they have other health-boosting traits.
Once you finally get the Administrator trait, there is a random event that gives you the Legendary Wisdom permanent modifier. This event is quite rare with a mean time to happen of 25 years. This means that the event will, on average fire about 25 years after you get the Administrator trait (note also that this is only if your ruler has 12 Stewardship or more, if they somehow have less than that the time is greatly increased).
After actively trying to get the trait it took me 5 generations to finally obtain it: my first ruler died a couple years after finally getting Administrator from depression, my second and third died before they even got that far (severe stress and poor health respectively; none lived past their forties). The fourth I had commit suicide as soon as his Midas-Touched son turned 16 which allowed him to finally live long enough to get it. He got Administrator fairly young in his late twenties and finally got the Legendary Wisdom event less than ten years later.