This achievement is very time consuming. You have to win every tier of rally races in the career until you get to the next to last tier. I unlocked the final tier with one set of races still left. The final tier is 5 stages, each with 6 events. So thirty races total. Those thirty races alone probably took me about 3 hours. A lot of them are 6, 7, and 8 miles in length. They are also a lot more challenging than the rest of the rally career. They throw all types of weather at you, as well as a lot of night courses. Just take you time and if you are having trouble beating the AI, you could always go back and lower the difficulty.
RoRoRacing also added the following to help you guys out:
Good guide, just wanted to add to please pay attention to your championship standings. The max points you can get for each event is 25, so if you get to the final event (6 stages) and you're leading by more than 25 points over the next guy, then you can start the event and then immediately retire from the event. You will place 30th in that event, but you will still win the championship and pop the achievement. I had 100 points going into the final event and the next guy only had 66. You'll see in the first few events who is coming in second, if it switches to another driver like it did with me, then you shouldn't have any problem getting more than a 25 point lead. That saved me from having to race the last 6 stages.