GRID

The first step to this achievement is completing 4 "Showdowns" which are the events at the very end of each category of races in Career mode. You can choose any 4 categories you want, but I recommend doing the GT category as one of them (more on that later). Once you have completed 4 of them, the GRID World Series at the top will unlock, which also gives you this achievement:



Next you must complete all of these races in the GRID World Series including the "Showdown: Ravenwest" event at the end, which unlocks this achievement:



Now exit career mode and go to Player Profile and select Team and Nathan McKane will be unlocked to buy as your teammate for $150,000. Now all you have to do is win a race using the Ravenwest livery. Go back to Career mode and near the end of the GT line is the Sunoco Prototype Super Sprint. If you already completed this before, you'll already have the car and before you start the race press to select a livery and under the Stock category select the Ravenwest Motorsport livery. Continue on and win the race and the achievement is yours. haugse23 points out that the Cadillac in this class has the livery available but the Acura does not.

You may also be able to win a race in Free Play using the Ravenwest livery but I didn't try that before unlocking the achievement. Also if you're trying to unlock all the achievements you will have to complete all the GT events anyway and buy the car for it.

The guide made by Erod550 is excellent and my guide is in no way meant to make away from or discredit his guide. It is just a supplementary guide of a list of cars that will work to obtain this achievement

The cars than can use Ravenwest liveries are:
Prototype class Cadillac DPi-V.R
Group 1 class SRT Viper GTS-R
Super Tourers class Holden ZB Commodore Supercar
F1000 class Jedi F1000-SPEC
Oval Stock class Jupiter San Marino Oval Stock
Super Modified class Mazda RX-7 Panspeed
Classic GT class Porsche 911 Carrera RSR 3.0