There are two potential ways of doing this - either boosting the achievement with at least one friend in an online Custom Grand Prix mode or you trying to get it in Grand Prix or Career mode with a 24-car grid of AI cars.
The easiest way of course would be to set up a Custom Grand Prix of 20% or more race distance, invite at least one friend, let him lead the race until the last lap and then overtake him to win the race.
If you are willing to do it in Grand Prix or Career mode, make sure to set the A.I. difficulty to Amateur and stay behind the leading car for the duration of the Grand Prix. As soon as you start your last lap, go for the overtake - with the A.I. difficulty level set to Amateur, you should not have problems overtaking the leader.