There are multiple quests needed to complete this achievement, and this can easily be done with the "Romance with Pointy Ears" achievement because all the quests are the same.
1. A Cure for the Prince
2. Idea For Traps
3. The Souls of Elves
4. Camp Security
(Either the traps or the materials to make them will work)
5. The Soul of a Prince
(this is not accessible until the Concubines are dead)
There's also a very important choice in Act 3 that's required:
After that, you should have the achievement when the final quest (The Soul of a Prince) is complete. As stated, this also lines up with the romance achievement, so female Vulcans can unlock it during the final quest as well.