EDIT: If you want to know the easiest way to get this achievement, look at Reborn Insanity's solution. If however you want to get it earlier, follow the solution below:
As soon as you start to see heat level 5 racers in free roam (should be around rank 10 if I remember correctly) enter a Hard Hot Pursuit event. Let at least one racer cross the finish line. You won't get gold for the event, but his heat level will skyrocket, usually to eight or higher. Bust him to get the achievement.
If it doesn't unlock, try it again - it might not work first time. It might be worth pointing out that when I did this, only one racer crossed the line, and he had remained first throughout the whole event. I also lowered his health a little bit during the event, to make capturing him after a lot easier.