Trouble missions occur randomly but in a chronological order. Once you finish one, the next will occur in time. Simply do 20 of them and you'll get this trophy. They're all very straight forward and usually only require going to the location on the map. The only exception is fighting a certain character who walks around town (like Mr Shakedown from Yakuza 0). There are 42 in the game so you can get this trophy done fairly early in the game.