Simply attack a town and defeat all of the enemy?s soldiers, then place a governor in charge, visit him and you will be promoted. Then leave the town and immediately attack it again and install a different governor, this will have the
effect of making that nation very happy and promoting you up a level. Simply repeat the process until you a a Duke with every nation. It also has the added bonus of increasing the amount of land that you own. Just make sure you have
enough troops to begin with.