I obtained level 30 after 8 seasons and most of the time i got A rankings.
It takes longer than i tought and i finished achievements like 50 objectives way earlier before hitting level 30.
There is no "special and super fast" method that save you much time, but i do my best to share my experience.
You got reputation for your ranking at the end of the race.
It seems a "triple" A ranking (pratice, qualifying and the race itself) DON'T give you extra reputation, because only the total! ranking counting towards.
So i decided to skip practice and qualifying and just finished the race to save time... but if you mess up the race, you earn in the worst case scenario not a single point of reputation... keep this in mind.
If you have an A ranking in the pratice and qualifying mode, you got some reputation points, even if you have no ranking in the race itself, but it saves time if you ONLY finish the race.
There is also no difference between a "nearly B" A ranking and a flawless race A ranking.
You got more reputation for your home circuit... so messing up this event is like messing up 2 events. Your home circuit is marked with a home symbol and you should do your best for a "good" reputation boost.
That's all i have to say about this achievement.
If you find a better way to obtain reputation, please leave me a comment! =)