In order to raise a banner you must not be serving under any lord. By doing this, you'll rebel against whoever owns the territory you have occupied meaning you have to face them in battle in order to obtain it. I'd suggest asking officers in the nearby areas to join you, free officers have a much better chance than those serving the lord of the area. If you don't hire some extra help the battle will be that much tougher given the fact it will be your unit alone. I'd suggest the earlier campaigns where the land is more divided, as the individual territories are weaker in these scenarios.