The unlock trigger for this achievement does not read from an in game checklist but instead actually reads the achievements from your profile. Most of the time this distinction wouldn't matter but for this game it, it's a saving grace.
This means that if you fell victim to the infamous save - quit - resume bug that plagues this title, you can delete your save data and complete only the missing achievements to force this one to pop. My 'unlock Yinglong' achievement bugged on my first playthrough and I was already 80% through gauntlet mode, so I finished everything else, deleted my data and played the game again just for that one achievement. When it popped, I returned to the main menu and 'True Warrior' popped.