Once you're a merc with a given nation, you can recruit elite troops from your nation's city commanders.
For example, if you're with the Polish, go to each Polish city and speak that has a commander, and recruit Winged Hussars and Polish Reiters. After 3-4 days, they'll be able to be recruited from the commander. So make your rounds and build up your troops, and disband your weak ones.
Once you have 40-50 Hussars/Reiters, you can take on enemies with 90-100 troops, maybe even more if you're good. Be sure that you take a look at what kind of troops they have before you engage them. Being outnumbered always gives you higher renown (if you win). If you're willing to wait, try to have nothing but Hussars (or Swedish Reiters/Black Reiters, or Nokhors, or Noble Guards).
That can build up your renown rather quickly.