1.) Build alot of militia and cavalry divisions
2.) Make sure to not research any small arms techs for them (makes them win faster)
3.) Make sure they have the leaders with nice traits you want to lvl up
4.) Attack a minor
5.) Use so as many militia divisions as possible, great if you overstack since that lower your attack and slows down combat
6.) Attack in the most unfavorable conditions you can find (across rivers into forts and mountains if possible).
7.) Upgrade your militia and cavalry into good units like tanks and motorized after you think they have enough xp.
Your units will take ages to win, but since your fighting a weak minor his units will also take ages to defeat your militia hordes.
Longer time in combat = more experience both for units and for generals.
Many players consider this cheating/gaming the system