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. You will have to play for a bit, until you get to the point where you take over Fort Dervingard.
2. After this, the next place you will take over is a settlement called "Loki Palace".
3. Simply attack, and you can auto play the attack, not having to do it yourself. It's an easy win, and the Palace is yours!