Progress by staying under the rule of an emperor and keep skipping months, slowly gathering money whilst ignoring any attacks that your faction may come under attack by and ignoring the merchants requests (he only offers cheap skills). When you come to have a reasonable amount of money, around 100000 is a decent amount (it may take a little while) leave the faction and move to a tile that has a stable. Once there your objective is to buy the Red Hare as this is the fastest horse and can be carried over. Then proceed to move to a tile where there is a blacksmith and use all remaining money on upgrading your weapon as highly as possible, this can also be carried over.
Ta-da! That's all the pre-requisites done, now it's time to use the method that some may call the ''late betrayal'', however, I just call it the method because fancy names sound stupid to me. This method is not extremely hard to grasp the concept of and it's fairly quick.
Firstly, you must be free of any factions, and just proceed to skip turns constantly until there are only two remaining - these will most likely occupy roughly the same amount of space. Join either one of these factions, and continue to skip turns, denying to defend whenever possible. This should result in your allegience losing land at a fairly speedy pace, wait until you have one or two pieces of land left. When this occurs simply leave and the following turn join the other allegience. Now you will be required to only fight one or two battles depending on the amount of land you left your previous faction with (one can sometimes be risky as it can get taken in your off turn). Win these two battles by connecting your main camp to theirs via a successive chain of bases, and the scenario will be finished. Save and you will gain the respective achievement.