My strat is to use Swadian Knights (the strongest troop in the game) to crush Swadia (the perceived weakest faction in the game).

Experienced Warband players will recall that I do get some hefty morale penalties for doing this, but if you crush Swadia fast enough troops only begin deserting at the very end of the run.

Summary of run:

The goal is to get Swadian Knights as soon as possible, as around 15 are enough to solo-cap most castles.

I started by leveling my recruits against outlaws (Tagia then Sea Raiders). When I got a handful of higher tier troops, I joined the Nords and took the fight to Swadia. With 30 battle size and reduced damage, I could take out their lords even if they were many leagues above my current army.

When I got a decent amount of Swadian Knights, I started capping castles. Nords recaptured and defended stuff I took for the most part. Thanks Ragnar. He didn't bestow any fiefs, though, so I had nowhere to put my captured lords.

This run can probably be cut by 20-40 minutes with the same strategy. I wasted a lot of time in mistakes and extraneous stuff.

It might be faster to take over the Nords (small nation, but has better troops) or Khergs (bad at defense, but field battles might be longer) but Swadian knights are probably the best as they are insane in field and in siege.