To unlock this achievement, you must complete Chapter 8: A Dawn's Light having at least 15 hours of playtime according to the timer in the main menu.
You can beat the game, and replay the level at a later time to unlock this achievement. I recommend completing the chapters, then working on Client Jobs and Side Quests within The City. After completing them, you can also idle your character in a safe location in The City to help build up time.
Once the menu timer reaches 15 hours, you can replay Chapter 8 by visiting the South point of Grandmauden. Once you complete the level, the achievement will unlock.