Image: Sucker Punch Productions / PlayStationOfficial screenshot from Sucker Punch Productions / PlayStation.
Living Legend is the platinum trophy for Ghost of Tsushima. You get it once every other trophy from the original game is unlocked, so it marks a full completion of Tsushima.
Clear the main story, every Tale of Tsushima and all Mythic Tales. Then mop up the collection, liberation and combat trophies on the list. The platinum unlocks the moment the last base game trophy pops.
Finish Body, Mind, and Spirit before you start New Game+, since starting it resets most of your collection progress. The 25 Legends, New Game+ and Iki trophies can be skipped entirely.