I used the Nomad GT (Supra, class 5, drift mod) on Castle Road.
To earn backwards DP you need to drift at angles above 90 degrees.
To earn a "mega backwards" you need to hold the backwards drift for 1.3 seconds.
You will earn the most XP doing mega backwards.
You earn 1,200 on 1x multiplier and 6,000 when on 5x, so this achievement can be done in less than an hour once you get the hang of it.
I went into a drift with a bit of speed and kept turning until the car was 90 degrees sideways then held the handbrake and pushed the left stick to the opposite side from the way you turned into the corner. If you hold it for long enough you should get a "mega backwards"
The video below shows me getting multiple "mega backwards" fast, you can see my controller inputs at the bottom right, and the angles I'm at in the bottom middle of the screen. It will take a few tries to get the hang of it but once you do they are pretty easy.
If you need me to explain any further then just ask in the comments, hopefully this should help you enough.
I had a better video showing multiple quick "mega backwards" but that got deleted, so here are a few put together.