First off, there's an option in Driving Aids called "Tricksteer", which is to supposedly help in Gymkhana events. I couldn't personally notice much difference with any of the settings, so set this to "Off".
My focus is to build a 2x multiplier from doing donuts. 2 donuts will usually get you there, or very close. Bear in mind that after the first donut registers, there will be a brief pause before the second begins. After that head on over to the purple 360 spin pad. Get as close to the centre as possible, hit the handbrake and keep spinning. This method will get you 3 gold coins easily on each career event. Just do a scout on your first run to find the dount ring that has a spin circle nearby. If you take too long between tricks, you'll lose your multiplier, so try hitting some boards or do simple drifts to keep the chain going between donut and spin circle.
For the achievement, I chose the first Gymkhana event "Cape Town Stadium" in the first chapter "Ignition". I used the Skoda Fabia.
The donut is just to the right of your starting point and the spin circle is just to the left.
After the jump, do your 2 donuts to get the multiplier up, the head off to the spin circle (try hitting a couple of boards on your way). Hit the spin circle as centrally as you can, then pull your handbrake and spin. You'll get 'Stale' appear after a few spins, but stick with it and you should get to 30,000 easily enough. The key is a 2x multiplier before you hit the spin circle.
In my video you'll see I messed up the start a bit getting the donuts to register, but got there within the time limit. Thanks to BIG BOSSDEN for the idea