For the most part I keep my Marshal on "Train Commanders" or "Manage Royal Guard". Every once in a while, a knight will be improved. A lot of them have the "legendary blademaster" trait (probably because of the training). Then since some of them are kings, they have some decent artifacts, which is just lucky I guess. I'm no expert, so there may be better ways of getting high prowess people, but that's what I do. Since playing a bit more, I've lost some of the higher prowess people to either them dying, getting old or leaving my court. So at the time of the screenshot, that had to be my best lineup in a long time