This achievement requires you to get a 9500+ rating and hold that rating for a good time/distance. From the time I hit 9500 and got the achievement, it was about 3-4 hours of doing hotlaps around Brands Hatch.
You will need TR(Track Competence), SA(Safety), and RC(Racecraft) to be 99. This will make the achievement much easier and may not require you to do competitive races online to get CP.
Easiest way to get your SA and RC up to 99 is to do 20 minute sprint races, then 45 minute sprint races, then doing 60 minute sprint races. Set the conditions of the race to light rain (run dry tires) and set the time of day to 9 or 10pm, so it is getting dark or completely dark. Set the opponents to 19, and your starting position to 20. Choose a difficulty that is comfortable for you, not too slow that you will be running into the back of the AI, but not fast enough that they are making you push the car too hard. You want to work your way up through the field slowly and cautiously because these ratings are not about winning, but about racing opponents cleanly. It will take a good amount of time to get these ratings to 99, and they will start to go up slowly every couple laps after they reach 80 or so. As long as they flash green every lap, they are going up.
Once you get your TR, SA, and RC to 99, you are very close, if not already there depending on your other ratings. For PC(Pace), this one is your ability, meaning how fast you are. With this it just takes practice, but the higher you get with this the easier it will be for the next part. Do all of the Special Events and try to improve on each of them because your best position on the leaderboards for these events is your rating.
Now you have to work on your CN(Consistency) and CC(Car Control). There is an easy way to get CN to 99 and to do so you just need to pick a car and go around either Brands Hatch or Monza in 1st gear, must be in manual, and try to get your lap times with in 0.1 seconds for about 5-8 laps straight. Your CC will suffer when doing this, so you will need to work on that next by choosing a track you know you can get "Green corners": Fine, Very Good, Ok but messy, and Perfect corners. You want to avoid losing control, going off track, overdriving the car, or going too slow. If you can get a couple laps in a row with Green corners then you should get around 85+ rating.
So you should try to get close to TR-99, CN-95+, CC-85-95, PC-70+, SA-99, RC-99, CP-0. If you can't get your PC up or you are struggling with SA or RC, then you will probably have to start running Competition servers online. If you get 30+ CP rating, you should try to grind out SA and RC again to 99, then do the 1st gear method to get CN to 95+, and lastly try to get CC to 85+ on your best track, probably Monza or Brands Hatch.
I got the achievement with TR-99, CN-90, CC-88, PC-88, SA-94, RC-99, CP-0 with an overall rating of 9554. I actually went from 9514 to 9501, then back up to 9554 when I got my achievement. You just have to stay above 9500 for a good amount of time before you will unlock Platinum.
I hope this helps, and good luck.