The magic journey to stop the Sun Gun happens in the third volume of the "Captain Wilkins" Freedom Chronicle DLC. There's as many open areas in this chapter of the DLC as there are enclosed space that, like normal, could either make or break your runs.
YouTuber "Kreeper V" 's guide is my musty trusty tool in my tool belt for doing runs of the Freedom Chronicle DLCs. This one's played on Mein Leben but if you can get the hang of it by watching the path they take and the precautions they take as well, you'll be able to adjust your run accordingly fairly fine.
Volume 3 starts at around "37:15" and goes until the end of the video. Be sure to keep you mix of stealth and gun-running blended thoroughly.