The Mental Block event is located in Crescent Mountains
in the upper portion of the map. Ken Blocks car is available for $0 in the garage after you've completed Manu's (Style)
Be sure to join a game by yourself. With other players in the game, there's a pretty good chance that they'll interfere, even if they're just free roaming. How to play alone:
button -> Settings -> Game Settings -> Play Alone.
Use the brake to initiate drifts and avoid using the e-brake too much since this slows you down more. Ken's car is fully tuned for drifting so it might slide a lot more than you're used to. It also makes it a little difficult to straighten out after a drift. Release or feather the gas to help control your car in these situations.
Try to get most of your score from the mountain sections in the first half of the course. If you can maintain a decent angle and keep your speed up at least 70MPH, you should be getting 10-20k per drift easily. Once you're in the city, there's less room for big drifts so it'll be a little harder but there are still plenty of opportunities for high scores.
Do your best to not hit any walls, especially in the mountain section at the beginning. Even lightly bumping the wall will cause you to lose all your points
for that drift. Bumping into other cars or traffic will not
cause you to lose your points for that drift.
You'll want at least 200k when you get to the tunnel. If you hit too many walls on the way down, it's probably worth it to restart.
Some people are unlocking the achievement just getting the required score and not having to finish the event, but does not work that way for others. It's a little buggy in that regard. Finishing within the time limit is really the only way to guarantee you'll unlock it.
As you can see in the video below, I do pretty poorly in the city section and still (almost) make it to the end and unlock the achievement, so there is some room for errors.