Else you'll need to complete this in co-op or via thefts. There's at least 1 challenge you'll need to complete before finishing all 47 story missions.
While playing through the main story, have a look at the challenges either ingame ( > Saintsbook > Challenges) or in this trophy guide with extra tips (“You're My Hero! ” trophy). You can play on any difficulty, there are no difficulty related trophies. Go through the story and enjoy the madness.
