This achievement requires you to reach level 50.
With the new update Ghost put out, this is a lot easier than before and if you're starting from scratch, you should earn this towards the end of your 100% completion. After I completed the story, all roaming racers, 3 starred all activities, and hit all billboards and found all the poker chips, I was at level 49.
If you've done all of this and are still in need of some levelling, you can go to any event you want, or one you found easiest, and re race it as I believe, after the update, you earn 275,000 rep for each win. What I did was go to the race "Fight Into the City", which is in the northern part of the map right at a gas station. This race takes about 2 minutes to do, full of straights, and once you're done you can simply fast travel back to the gas station and do the race again right away. This is what I did to get the last bit of rep I needed.
Also of note, I'm not too sure when it starts, but in the 40's, you're going to need about 1,000,000 rep to level up. With the new update you shouldn't need to grind too much if you're going for 100% completion, so I would recommend saving this as your last achievement once you've done everything else and grind it out with the race mentioned above.