You must have the "Hammer of Infinite Fate" DLC for this achievement. You must also have completed the game in order to make out with Ophelia. The "Mountain Man Threads" can be purchased as Threads for Eddie in the Motorforge for free. After equipping them, you can find Ophelia at the large Iron Cross landmark, just west of Bladehenge. Simply interact with her and after the cutscene the achievement will unlock.