This should be one of the first achievements that you unlock while playing through the campaign, so long as you don’t join another faction too soon. However, you can always get this one done much later in the game after you’ve already joined and then left a faction. This is the natural progression if you want to work towards establishing your own kingdom. At this stage you should have a sizable force that you can use to contend with other armies and, ideally, defeat them handily. The key here is not to use any allies, and fight only with an army consisting of your own troops.