To be clear, you need the starting three characters (Jadwiga, Krzysztof, and Kazimierz) to reach Day 63 alive. This needs to be done in one playthrough so I would actually recommend doing it on your first one. Start your very first game on Easy/Easy difficulty and focus your efforts on winning every mission and keeping your districts alive. Easy/Easy will likely still have you reach Day 63 with 1-2 districts intact. Please refer to my guide for the "63 Days of Glory" if you are concerned. That guide contains extensive tips/tricks to ensure a good ending.
There are a couple things to watch out for.
1. If the health of any of these characters is low after a mission, sit them out for the next mission. Jadwiga is a great character and will frequently get used based upon her healing abilities. Likewise Krzysztof has excellent attack ability between his rifle and heavy weapon. Just watch out on missions that you don't let them get to low or a lucky crit could end your attempt. Dashboarding might give you a better fight, but best not to let them get too low on health to begin with.
2. Avoid in-mission encounters with these characters. You might pull a bad one, and they get insta-killed.