Just a note: you will have to complete the quest lines for both Shaundi AND Fun Shaundi to pick up this achievement. Only completing the modern-day Shaundi's quests will not net you this achievement.
There is, however, only one Shaundi loyalty mission, and you can only romance modern-day Shaundi. The only portion you will have to double up on are quests.
This includes romancing Shaundi on the ship (The modern day Shaundi) - I had missed this and completed the game getting the fun ending and was wondering how I'd managed it.
-- Partially borrowed from TrueAchievements