Image: Sucker Punch Productions / PlayStationOfficial screenshot from Sucker Punch Productions / PlayStation.
There Can Be Only One is a combat trophy for winning every duel. Duels are one-on-one fights spread through the main story, side tales and Mythic Tales.
Finish Jin's Journey, the Tales of Tsushima with duels and every Mythic Tale with a duel. That includes The Heavenly Strike, The Legend of Tadayori and The Six Blades of Kojiro.
Duel opponents mix in kicks, shoulder charges and attacks you cannot block or parry. Dodge those and parry the rest.