Very easy to get.
Hit up Free play and choose the 1966 McLaren M2B from your cars. If you have not purchased it, you can rent it.
Choose Homestead - Speedway Circuit as your track and then go to your game options and set your Max Drivatars to 23 for a full field, drivatar difficulty to New Racer or whatever difficulty you are comfortable with beating them, set laps to 1 and collisions to off.
Once you start the race, you will start near the mid/front and with collisions turned off, you can drive right through the field into first. I did this very easily on Homestead because you can finish the lap around 35-40 seconds.
Achievement pops when you finish in first.