DiRT Showdown

For this achievement you need to beat the pro final, which is unlocked as the last event in the 'spy+ pro' group of events. It is a race-off event and all you need to do is come 1st, 2nd, or 3rd to get the achievement. I recommend trying for first, as it will make things easier when you're going for gold in every event. Beating this event unlocks the DC group of events, so this achievement is story related and unmissable.
Struggling? Try changing the difficulty to casual in the options, and don't forget to use boost on the straights! Also, I find in most events that keeping in the middle of the road helps as you won't crash into the signs along the side of the road.

Here is a great video with spoken guide from savgcom on YouTube. Hope it helps you out! He's got some other video guides for this game, so his videos might help you with some other achievements.