This achievement really just requires time to unlock. When each of your rulers die their accumulated piety and prestige will be converted into your score so play long enough and you'll eventually get it. It took me about 200-300 years to get playing naturally starting as the King of Svipjod in 769. Alternatively, if you have Horse Lords nomads are a stupid easy way to get prestige. Just conquer everyone around you and keep the land for yourself, revoking non-nomad titles if you end up with them (the clans don't care) and raze every holding to ground and you'll get tons of prestige and enough score in no time. Playing as the Uyghurs in the Charlemagne start I got 100,000 score in about 80 years with only five rulers.