Easiest to do with Haesteinn of Nantes in 867. He starts with 3k men. Start by going to war with Nepoli once you have Nepoli go to war with the island right under the Byzantine Empire, then move on down south too Naquis and go to war with them once done you are close enough to reach Socotra and once you take that click on the castle and use the move realm capitol button to move your capital and the achievement is yours.
You only need to take counties no need for taking whole kingdoms and duchys for this achievement unless you plan on continuing your playthrough as him. You could also get the max level of prestige achievement if you have not gotten it yet with him as he starts with almost max level and can be reached with only a few battles.