You can achieve this during the "Ring & Remembrance" quest. It's part of the main storyline. After blowing the rubble from the armoury stairwell, head through the newly opened area and into the "Northern Slopes" area. Work your way all the way down the hills until you reach the sea shore. The achievement should unlock once you reach the shore. See video for exact directions.
Note: this achievement is technically unmissable but you won't achieve it until the very end of the game. You can unlock it much sooner after the aforementioned quest.