Here's a video on how to obtain ALL of the prologue achievements, and more! Links to the specific achievements, in order of when you'll obtain them, are in the video description. I didn't show battles as some are walked through by the AI, and most others are simply auto fought and won. The last one appears very difficult, but my game cut out, and I didn't complete the prologue.
As for this achievement:
1. After Loki Palace, you will take over another fort just North East of you.
2. Next to this fort will be a quest battle, that is a beam of blue light around the quest.
3. Do this quest for this achievement.