This one is worded poorly. I'm assuming you're supposed to drive the Centenario to win the Sesto Elemento. If it turns out to be the opposite, I'll change this.Blueprint a short, quick championship and jump in your Centenario to knock this out quickly. If you don't own the car, it's 2,300,000 at the Auto Show. You can get them on the Auction House for significantly cheaper, as low as 500,000 if you're lucky. So grab one there and hammer down.
Between this and
You should have the majority of
EDIT: Apparently both the other solution writer and I are getting negative votes due to this not unlocking for some people. The Forzathon challenges are known to have problems unlocking and over 120 people, including myself, have this unlocked. So if this doesn't unlock for you you may just have to retry. Or try restarting the game and/or your system. Unfortunately I've never heard of any reliable way to make these pop when they've glitched.