How it's done, done, done

Needs All Under Heaven, and the Unite the Husamguk decision is only available to the Buyeo culture group from the 867 start - so play Silla, 867. Handily, the one county you're missing starts independent: take it, grow the realm to the decision's threshold, and take it. Korea united, achievement done - and once Tang China starts wobbling, the same campaign can wander into the Chinese achievements (Stamp of Approval and friends).