Make a young martial-focused character your next ruler, and take him on a tour of all the warrior lodges. Pick up the rank 2 commander trait from each one (as well as 2 normal commander traits, if possible), aiming for a
minimum of 5-6 commander traits. (8-10 commander traits would be ideal.)
- Warrior lodges require HF - not sure if you have this.
- Don't ever become zealous while a member of a wrong-religion warrior or you'll be kicked.
- To switch lodges:
- Ensure you have {250 piety + 1000 prestige} or {500 piety}
- Leave your previous lodge
- Wait 90 days
- Pause
- Secretly convert to a pagan faith from concubine / holy site (costs 250 piety)
- Personally adopt faith
- Apply to new warrior lodge
- Convert back to original faith. (If you have RoI you can convert to your capital county's religion - costs 1000 prestige. Otherwise, do the secret conversion thing again.)
- Unpause
During this process, it's probably worthwhile to throw your leader into battles as often as possible, as battlefield events can seriously improve your character - eg. brawny, patient, martial education upgrade.
There are also some artifacts that buff combat stats. If you have choices in your loadout, I suggest you prioritise boosting PCS to 100ish (to ensure you nearly always win battlefield duels) then boost martial (because martial multiplies all army combat stats) then boost individual army combat stats (morale, damage, etc).
After all that, any armies he leads will be able to defeat MUCH larger armies (depending on troop quality and terrain, anything from 5 times bigger to 15 times bigger): move all the subunits into the same flank (eg. centre if you have "Direct Leader") and put your ruler in charge. Opposing armies will literally melt.
This approach does have some disadvantages. First, it requires a LOT of setup time. Second, you have to continually focus on levelling up in the societies and your character can't focus on much else. Finally, your character can't be in two places at once. Yes, he can instantly teleport between armies, but you still need to be careful to ensure that he's not needed in more than one battle at the same time.