You must have the lowest Index car in the race and then win it. Each car has a class and a corresponding number beside it. The lower the number the worse the car. For this achievement, you need to do a race where you're car's "number" (PI Index) is the lowest and win. My advice is to beat one of the first events where you win the Porsche 1970 416 (Index 196) Don't add Parts! Take this car and use it in the European race under proving grounds at easy difficulty, and you should be able to win. This car will also be useful alter on, but don't upgrade it.
Note: Can be done online with a friend very easily.