This is an achievement that can be unlocked through natural progression, HOWEVER you have to willingly start the event to do it, as the event is not mandatory.
In order to get the Horizon Champion achievement, you must first enter and win 15 Championships. If so you wished, you could just repeatedly enter different championships in the same city, but seeing as you're going to have to play through all the locations for achievements anyways, why not just switch cars as you go?
Use a car from a different championship class each time. So if you choose the Supra in the beginning of the game, use it for the first Championship, then your free Ferarri 360 Challenge Stradale for the second one, then buy a different class car for the third (By the time I got to Nice I already had enough cash from online play to buy an Aventador and a C7 Corvette) and onward until you've won 15 different championships.
After you win these fifteen championships, you should be able to enter the Horizon Finale to unlock this achievement.