Keep an eye out for available Missions and Requests

I know this one sounds obvious. Like, of course you’ll get stronger if you complete optional missions, welcome to virtually every video game ever made. That said, Dynasty Warriors: Origins doesn’t always make its side objectives obvious—and you’ll want to complete all of them if you’re looking for total completion and a shiny platinum trophy. Even bond events can lead to optional challenges you can complete for an infusion of skill points, and most of these are easy to accomplish in the background while you play the main story.
To make sure you don’t miss anything, I recommend opening up your map screen after major story battles to see if anything new has popped up. Also make sure to keep checking back at the inn to see if you’ve gotten more mail, since that can lead to more optional content as well.