Crusader Kings III

An easy method for this is to start as Infanta Urraca in 1066 but have the setting "Gender Equality" set as "Inverted". This will have Urraca auto-inherit all three kingdoms (Galicia, Leon, and Castille) immediately.
Marry Rodrigo 'The Campeador' de Vivar and butter him up with Sway/Seduce/Romance so you can get an heir.
You can struggle clash your way south east towards the kingdom of Valencia and conquer 6 of the required counties to make the kingdom title.

Remember to make the Kingdom of Valencia title your primary title so it will get passed down to your heir. Once you die, the achievement should pop

If you just want to get this achievement done as quickly and easily as possible, Zekling's solution to play as Urraca Jimena with inverted gender game rules (which instantly makes her inherit the kingdoms of Castille, Leon and Galicia and moves El Cid to her court as a knight) is the way to do it.

If you don't want to cheese it with weird special game rules, here is how to do it with standard rules:

We need to get a player heir who is also a descendant of Rodrigo de Vivar aka "El Cid Campeador". The game shortens his nickname to "the Campeador", in case you tried to use the character search with "El Cid" and wondered why no one showed up ;-). He starts the game in 1066 where he historically was at about that time: as a landless knight in the service of King Sancho of Castille and a friend of Sancho. If Sancho gets killed and his realm gets absorbed into another Christian royal court, he will offer that ruler his services (happened to me once playing as Alfonso after I murdered Sancho without getting caught, though I'm not sure whether El Cid would also have offered his services if I had been caught at murdering Sancho).

The achievement says in game that it can't be done "with some custom-created rulers", which probably means editing your starting character as a female catholic duchess of Valencia will not work. The only historical female Christan ruler in Iberia in 1066 is Urraca Jimena, but I wouldn't recommend starting as her, since she has a very difficult start as a vassal ruler of only one small county and marrying a landless knight instead of scoring a strong marriage alliance makes her playthrough even tougher. El Cid is a god-tier knight and general, but he won't help you much in conquering Iberia if you don't have an army for him to lead. Playing as Urraca may be the quickest way to get a descendant of El Cid as your heir (though King Sancho needs some buttering up first before he agrees to marry his friend and best general off in a matrilineal marriage to his rival's vassal and Urraca's biological clock is ticking), but the part of conquering Valencia is quite tough to do with her.

For an easier game, best play as King Sancho himself. Since Vivar is a member of your court, you can search a suitable wife for him. Since of course we want the wife move to our court, not El Cid to move away to a foreign court, make sure you pick someone who isn't landed, employed in a court position in a foreign court or in line to inherit titles. In the ideal case, she has also a good inheritable trait. Also have a look at her character traits and make sure they are compatible with Vivar's, the honest brave knight might be reluctant to breed if you pair him e.g. with a deceitful craven.

I recommend not to land El Cid right away. He may get fewer kids when he isn't landed, but when he is landed, he'll eventually get into his own struggles and might get himself killed and he also might search for suitors for his children himself and might get picky about the marriage of his firstborn. As long as he is your unlanded courtier, he'll agree on any marriage terms you propose for his kids, including marrying his firstborn son into a matrilineal marriage with your firstborn daughter, in case you should produce only daughters and/or he should produce only sons. If you want to make 100% sure that he'll produce a suitable child to marry to your heir, you can keep him out of battle until he has done so, but the risk of losing him in battle when you use him as a general is very low due to his god-tier prowess, as long as you don't send him into battles in which your whole army gets wiped.

Sancho is the most powerful military leader in 1066 Iberia and it is usually quite easy to get the dominance ending of the struggle with him during the first hostility phase. All you have to do is keep conquering while employing a good loyal spymaster to prevent Alfonso from murdering you. By the time your grandchild is ready to take over, you should have the counties of Valencia already in your hands. If you can create the title during Sancho's lifetime with his primary heir still alive, you can give the Kingdom of Valencia directly to your grandchild and conclude an alliance with him to protect him if necessary, otherwise just don't create the title until your grandchild takes over, to avoid it being passed on to one of your other children.