I've heard many people had problem to get this achievement so i decided to make this simple guide.
How to Obtain Winter Soldier Achievement (Step-by-Step)
First select "Private Testing". For some reason this did not work in custom race for me, unlike many guides which say the otherwise.
Then select the correct track "Nurburging Nordschleife". I used Audi R18 e-tron '14.(it's not a DLC car.) You can have all the assists on.
The settings i've used to get this achievement are those in the screenshot. The race track doesn't need to be in snow, just the weather forecast to be "Snow".
Drive a clean lap without going off the track and the achievement will pop up as soon as you cross the finish line under 8 minutes.
That's it you're done.