This is a lot easier than it sounds. Thanks to
NightHawkJoV for his solution on the 360 version.
So, head over to the junkyard and get a speed car. If you select
Legendary Cars and scroll over to the 3rd car which is a
Carbon Carson GT Nighthawk, select it.
Now, head to the South East corner of the map towards the baseball stadium and find a stunt run event (green circle). Once you start up the event, there will significantly less traffic for you to hit. Head over the highway highlighted in yellow on the map and drive into oncoming traffic. You'll constantly build up your boost and just hold
all the way. You'll only need to avoid the odd car here and there and toll-booths. Once you hit a 20x boost chain, simply stop the car and the achievement will unlock. Good luck.
Update: Recorded and added a clip of me using said car and event to get to 20x burnout boost bonus
Update 2: As
JACKKAV points out in the comments below, if you're using a lower end Speed boost car, you'll need to get a couple near misses after every burnout. If there are no cars around, simply tap the brakes and get into a drift.
DO NOT let go of the accelerator
and/or boost
buttons as you'll lose you're burnout streak.