A couple of Swedish Armies appear in Mount&Blade: With Fire & Sword during the Polish storyline quest The Deluge.
Every Army numbers 250 men when spawned. It has around 70 Swedish Reiters, 100 Musketeers and 80 Pikemen. This is a very formidable force, with high ranged firepower and good defense from the pikemen.
Battle[]
Armies patrol around Warsaw and Krakov (Swedish at that time) trying to kill any Polish party. When it is destroyed, it will respawn at full strength.
You shouldn't try take down this army head-on, even if you have several lords by your side, as it will respawn a week or so later. If it is unavoidable, you should try charging at it with heavy cavalry like Winged Hussars or Armored Cossacks and target their musketeers, then rally your men and order your musketeers to shoot at the pikemen with no ranged weapons and the Swedish Reiters who have pistols.