This achievement is easiest in "Write Your Own Story" mode. Set the Cease Fire to 20 days, and set Winter to Mild and Short. When choosing your map, make sure to choose locations with the least hostility/danger. I also found it best to only have 1 survivor. This way you only need to keep the 1 person alive and don't need to worry about additional mouths to feed. Once I got to day 15, I had so many supplies, I was able to End Day and Sleep/Stay in the last 5 nights to rush this.