This achievement is only achievable within the career mode (GRID World) like most other achievements in this game. Events contain between 1 to 5 races within them, to get this achievement simple win every race in a single event, this can be done easy early on in the career mode. If your still struggling to win all the races, try to pick events within a very low number of races. Once you've won the last race in the event (given that you've won all of them) and complete the event the achievement should pop.