Need help

I just finished the game, going for the platinum trophy. I’m confused, do I have to start a whole new save file to get the saint trophy or can I get it on my level 50 character on playthrough 2? Whenever I go to start a new game and select a level 50 save file it throws me in on day 1 at level 50, I’m just wondering if can start rescuing survivors on that file or do I have to start a new one at level 1?