In order to net this achievement you have to reach level 500. First you want to master every event (all 480) with the different groups. If you still shy of getting to level 500 just start doing the online for the game cause you'll get XP for completing races either doing road trips or the playground games. If you're still shy of hitting 500 just keep replaying the Horizon Finale race over and over again. I'm using the BMW fully maxed out and its taking about 8 minutes each time. Good Luck with it cause this games is now an ultimate grind and a lot harder than FH1. This is where I have my source. Someone on the site was using a modding program to see.:
http://www.thetechgame.com/Forums/t=6777545/forza-horizon-2-...