Best way to get this is with a boosting partner. We did it like this:
Choose 1 lap in Curitiba, i think it´s the shortest and easiest track and choose F3000 cars. When you get to the qualifying lap just drive into the pit the wrong way. You will both get disqualified and save a lot of time. But even then the 50 wins (with 2 players it´s 100 races) will take a long time, just be patient and watch a few good movies while you´re waiting.