Total War: THREE KINGDOMS

I personally found combat tends to give out the most XP, but you can also level up characters through assignments and events. Administrators also seem to get some amount of passive XP, and there's a council action to give a random character 200% XP rate (it only seems to want to select lower level characters for this, though). You can boost character XP rate even more through the private tutors and treatise on human beings reforms, both of which unlock higher levels of the schools building. Ancillaries that boost XP rate exist, but I didn't see any of them in my last campaign and only a couple across all of my games. This isn't the longest of the achievements but it'll still take a fair bit of time to complete. Side note, completing this achievement should send you well on your way to completing A Sharp Point Sticks Out as well.


Edit: Just adding that assigning a Grand Tutor to your council and spending prestige points on assignments can also increase your XP rate. The Grand Tutor position is unlocked when you reach Marquis rank. I know some factions have unique council positions as well, but I don't know the effects of all those positions currently.