Swadian Knights are my favorite thing to kill. I can't play as Swadian because I hate the look of those horsemen in red and their flat-top hats. I conquer the map with Vaegir Knights multiple times. I don't even bother using Vaegir infantry.
You can play as any favorite faction, mix and match your troops, etc. but the key is to level up your party skills in Surgery and the other medicals, and to level up your party skills in pathfinding so you can rapidly deploy. Primarily use cavalry. That's the reason I like the Vaegirs because they are good on both horseback and foot.
Once you have your own kingdom, think about who you give what fife and strategically where. I give away all the villages, but after I have built messenger post, etc. After you give it away, you will still get the notices. Keep most of the towns and the siege tower castles for yourself. Try to keep the vermin lords from burning your villages so your economy doesn't go to hell. Release them often so they become *your* vermin lords and burn your opponent's villages. My completion goal is 1000 game days to win it all but last one took me 1100.