This was my last achievement and thought I could save people some time on this achievement with some tips.
Firstly some info.
This is NOT best done in career as the A main is the only event where the wins are counted as heats & qualify dash races do not count which is odd.
Secondly quick races also take too much time as you need to do heats, qualify dash to get to the A main and don't count either only the A main race.
I suggest you do the online wins because those 10 count towards this achievement also.
After 3 seasons I was only at 66 wins and simulated the other 7 seasons but even with a fully upgraded car you never finish inside the top 20 when you sim races, so that was no help.
So I did it in splitscreen, so you will need another controller obviously. I chose the Sprintcars and used my custom car that was fully upgraded (its works fine with a normal AI car) and chose the track Millersburg which is a mile long. Once it loads press
twice to skip practise & qualifying, so you will start in 24th and what you want to do is hug the inside of the track even going on the grass as it doesn't slow you down, and by the end of the first lap you should be in 1st place or in the top 3, and then be in 1st by the first corner of the second lap.
There is only 2 laps, so when you come onto the main straight on the second lap let go of the accelerator and pick up the second controller and hold the accelerator so player 2 finishes because the second players car stops 100m from the finish line, and it will only take 3-5secs to cross the line and finish the race.
From there press
twice and when on the EVENT END screen press
to race again, now just skip practise & qualifying and you won't have to worry about loading screens again saving even more time and continue this method race after race.
I was averaging 55-59secs per race and after an hour is was getting around 40+ wins.
Now for the achievement to pop you must finish the event and head back to the track select screen when you have enough wins, I was finishing every hour just to make sure the game saved the wins. and you can look at the tracks stats on the right side of the track select screen to see how many wins you have.
Other tracks may work also but I found most were just too short. Good luck.