It's pretty easy to get max ranking, just get 90+ off the Portrait Ghosts' meter in one go for their Big Pearl, look around for treasure ghosts (Blue Ghosts / Gold Mice), water plants, etc. Really just takes a few runs and taking the time to explore, and a smidge of practice catching the ghosts.
I generally enjoy Fire Emblem but I'm not very good at it, and being a perfectionist who doesn't want his units to die end up resetting a lot. So runs of those take too long for me to bother replaying them.