This achievement is simple enough: get all gold medals in the Factory-Spec Races.
One little thing that can give you an edge is that even though you can't put any upgrades on your car, you can put on new wheels to make the car lighter. Yes, it may only be about 30 pounds less, but it's still an advantage.
The lightest wheels are:
The first model under "Enkei"
The first model under "Motegi Racing"
The first and third models under "Volk"
The first model under "Work"