Crusader Kings III

This achievement comes with two major challenges:

1. Surviving the early game phase, because you have to start as an underdog chief surrounded by other tribal nations who'll constantly try to raid and conquer you until you manage to become stronger.

2. Not getting bored during the tedious waiting game that is converting the culture of 29 counties.

Fortunately, I have found a shortcut for the latter, but it still takes quite some time during which you don't have much else to do, once you established a powerful stable realm. I started in 867 and was done by 1007.

TLDR:
- start in Fika with custom created martial character who is a good general
- expand fast until domain limit as a count, than raid and defend until you can diverge culture
- after diverging, get higher rank and grow realm quickly
- start culture conversion with the Sao counties
- conquer three holy sits of Bori, reform Bori to be fundamentalist and have Communal Identitiy and Mendicant Preachers tenets
- convert counties to your culture until you have 30

1) What we have to accomplish

To get the achievement, we need to start the game as the chief of Fika, a county in Subsaharan Africa, part of the de jure kingdom of Kanem and empire of Kanem-Bornu. You can (and should) create your own custom ruler for this, you just need to be the chief of Fika and you need to keep the default culture (Sao). You then need to create a divergent culture (not a hybrid!) BEFORE achieving any title of higher rank than count/chief, because Fika still needs to be your primary title when you diverge your culture.

After that, you need to grow your realm and convert counties to your new culture until you have a total of 30 counties of that culture in your realm (The counties that initially convert culture with you when you create the new culture do count for the achievement, so you just need to have 30 total, not spread it to 30 more.) The 30 counties need to be in your realm (the shortened x-box achievement description fails to mention this, but the ingame description does), so make sure to retake any converted counties that you lose in case your realm breaks apart due to sucession or rebellion.

I have not verified whether it would make the achievement impossible if one of your ruler characters has another culture at any point in the game (if not, it would still definitely cause delays), so don't grant your player heir a county that still has a different culture as their first title (which could cause them to convert to the local culture).

2) Surviving the start

Fika is small, weak and the initial Sao culture is very underdeveloped and is a rather peaceful peasant culture with traditions that aren't useful for tribal conquest at all. You are surrounded by other tribal nations, many of them stronger than you, so you need to expand quickly and you absolutely need a brilliant general to lead your troops. Since your petty little chiefdom at the edge of the map likely won't attract too many skilled courtiers in the early game, you should edit your ruler to BE that brilliant general. Give him a good martial education trait, stewardship 12 (so he can hold at least 4 counties himself without penalties), decent prowess for staying alive in battle and the intelligent trait for quicker lifestyle experience progression and better heirs. Also consider making him a witch, because the local Bori religion regards witchcraft as virtuous and being able to create a witch-coven with the vassals from your dynasty is very useful in the mid-game for quickly picking up skills. Learning shouldn't be his absolute dump-stat either, because we do want decent progress (The first innovation I went for was mustering grounds, which allows you to increase your men-at-arms and thus gives you a huge military advantage over other tribal neighbors who haven't unlocked this yet).

First lifestyle perks to pick up should be "serve the crown" for quicker control gain and the four rightmost perks in the chivalry skill tree, which will increase the effectiveness of your army immensely during the early phase and will enable you to beat enemy armies that badly outnumber you in levies.

The starting situation is basically the same no matter whether you start in 867 or 1066, but I recommend starting in 867, because you are not just AS far behind more powerful cultures in technology by that time and it will take longer until powerful Islamic empires push into pagan Southern Africa and steamroll everyone, so you have more time to catch up in development and build a strong realm or even be done with the whole achievement before that happens.

At the start, you won't have quite enough fame to diverge your culture at once, so your first priority is survive. With that small county, you'll probably won't get a good alliance at the start, so rather go for an intelligent lowborn wife with good stats and start breeding good heirs right away, you'll get better alliances by betrothing your kids once you climbed up in rank.

Use almost all the starting honour you have to increase your men at arms to the maximum. Don't be afraid to have a negative monthly fame gain, you'll gain fame by constantly fighting in the early phase. First conquer 2-3 more counties, starting with your fellow independent Sao/Bori counties to the east (using conquest casus belli against fellow Bori will cost piety instead of fame, so we can save all of the fame for better troops and for the culture diversion). Unfortunately, having more counties will also make diverging your culture more expensive, but we need to grow to get more levies quick and at least we get the chance that more than just our initial county will convert culture with us. Don't go too much above your domain limit, though, because you need to stay a count until you diverge your culture, so you can't give counties to vassals yet.
Now we need to get a lot of fame points to diverge our culture. The quickest way to get those is fight battles against armies of about equal or higher numbers, using our greater number of men-at-arms, our ruler's high battle advantage and the increased number of knights from his chivalry perks to win. We can't conquer more counties until we rank up to high chieftain (else we would lose too much of our levies to the above domain penalty), so just raid your neighbors and/or succesfully defend against their invasions, which will likely occur at that time. Save up gold for creating titles later.

It is tempting to try and replace at least one of those agriculture and city dweller traditions with something more useful (like e.g. philosopher culture) during the diversion, which would establish the new traditions instantly, but unfortunately that is prohibitively expensive in our situation. It would take a long time to gather that much fame by raiding and battling as a count and there is a big risk that one of our neighbors snowballs to be a powerful kingdom that can crush us if we delay our own expansion for that long, so I guess it's best to diverge culture as soon as possible without changing traditions.
What we can change without additional costs during the diverge is basic ethos, language and gender roles. Since Bori religion already has an equal gender doctrine, it is useful to switch the culture's martial gender to both, which enables to use women for all positions. We'll likely burn through quite a lot of knights during our initial conquests, so doubling the pool of potential candidates will come in handy. I also recommend to switch the cultures basic ethos to either egalitarian (for quicker culture acceptance growth and better foreign culture opinion, but it doesn't help with culture conversion) or buerocratic (for quicker lifestyle experience and cultural fascination gain). I went with the latter.

After diverging your culture, create your first duchy title and quickly expand to get your first kingdom. Either Sao or Kanem is the logical choice. NOW you can try to get a good alliance with one of your stronger neighbors for additional safety. During the mid-game, you probably should also create the Empire of Kanem-Bornu. This isn't strictly necessary and it takes 44 counties minimum, so a bit more than we need for the achievement, but it helps keeping your realm together upon sucession, since we are stuck with tribal partition law and getting more than 30 counties would most likely lead to having enough counties to create additional kingdoms.

3) How to convert counties to your culture quickly

Converting a county to your culture is a councillor task for your steward and the speed will mostly depend on their stewardship skill, but even a steward with skill beyond 20 will usually need about 7-10 years to convert a county, depending on the county's development (high development makes culture conversion harder, but most counties in that part of the map have very low development) and possible resistance perks of the target culture.

You get a big conversion speed bonus for converting counties of the culture you "recently" diverged from, so conquer all Sao counties and start converting them right away (unfortunately those are far less than 30). I don't know how long exactly "recently" lasts, but it is at least several decades, I was able to convert all Sao counties before the bonus expired.

To speed up the later conversion of the counties of cultures other than Sao, you should try to seize 3 of the Bori holy sites and save up piety to reform Bori religion. (By going on pilgrimages and by fighting battles against troops of a different faith). Once you have all the martial perks you need, switch to learning lifestyle and try to get to the prophet perk, which will half the piety cost to reform your religion.

Once you meet all requirements for reforming your religion, reform Bori to become fundamentalist and to have the "Communal Identity" and the "Mendicant preachers" tenets.

The Communal identity tenet will give you a huge 50% bonus to culture conversion speed in counties whose population shares your religion. The downside is that it also gives you a 50% malus to religion conversion in counties of a different culture, but the speed boost from mendicant preachers and fundamentalist makes up for that. More important, being fundamentalist ensures that almost all of your same religion vassals will use the religious conversion councillor task, so you just have to convert or exchange your vassals and your whole realm will be Reformed Bori in few decades, giving you the 50 % bonus for all the culture conversions you need to get done for the achievement.

[Alternatively, instead of reforming Bori you probably could just convert to a religion that has the communal identity / chosen people tenet and is fundamentalist or at least righteous to get the same result in the end (religious conversion will be slower without mendicant preachers, but since your vassals will be helping, it will still be much faster than the culture conversion). Conquering three Boori holy sites isn't too difficult, though, since they are quite close to your starting point and they do provide some nice boni if you have them.]

Unfortunately, there doesn't seem to be any way to get your vassals to help with culture conversion. I don't think I ever witnessed an NPC using the convert culture councillor task. They always either promote acceptance, or they convert to the local culture themselves or create hybrid cultures. So you have to convert the culture of those counties one by one until you have 30.