So i've found a few really effective ways to boost character levels quickly that work pretty consistently. The academy in ambition mode is probably your best bet once you get it up and running, since you can just dump gold to instantly level characters up to 100+ for about 220k gold which is pretty nice if you've got the resources saved up.

If you're looking for something more hands-on, there's this great spot in wei's hypothetical route at baidi castle where siege weapons keep respawning endlessly. Grab someone with fast attacks like jia chong, slap quick learner on them, and you can rack up experience really quickly especially on easier difficulties where you won't get overwhelmed.

Something interesting about maximizing your gains - higher difficulties like hard or chaos mode will give you way better experience rewards, and there are a few skills that can really help speed things up. Quick learner is the big one since it doubles experience at max rank, but sprint-growth and victory cry are solid picks too. Using weapons with frost poison or flame effects can help you take down officers faster, which means more experience in the long run.

For material battles, bringing along bodyguards with fire attacks makes crowd control way easier, and if you're planning to tackle tougher stages you might want to spend some time crafting better weapons through fusion first. The whole process flows pretty naturally once you get into a rhythm with it.