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. I can not be certain, but I believe this is after the quest within the blue light, and just north of that.
2. This would be the second quest with a blue light around it. It was a very hard battle, and I wasn't going to be successful when my game crashed.
*I'm not sure if this is 100% right, or if there's a way to get both Lords, and their armies, into this battle. If so, it would be a much different scenario. Let me know your thoughts, if i'm right/wrong, and what to do!*