The quickest and easiest way to get this achievement is to boost.
There's a race near the north-west corner of the island, at the eastern end of the air strip (you'll see it quite easily on the map). The race is called "Near Future" and is a simple, 1 mile drag race with no traffic. If you use a very fast car, such as a fully tuned Saleen S7, the race should take 20-23 seconds. Taking into account the loading times, it should take you 20 to 25 minutes to win 30 races.
I would strongly recommend that you find a boosting partner because the majority of people who still play the multiplayer races are those who are hardcore into the game and only race to be at the top of the leaderboards. As a result, there's very little chance that they're likely to let you win.