Those fortunate enough to have been playing the game during the U.S. Thanksgiving weekend will have gotten the cars at half price, but the first step is to obviously purchase the Ferraris. Here are the ten I purchased (was going for cheapest, but already had the F12berlinetta):
2012 F12berlinetta
2010 599 GTO
2009 458 Italia
2008 California
2007 430 Scuderia
2003 Challenge Stradale
1994 F355 Berlinetta
1991 512 TR
1987 F40
1984 GTO
Go to Track Select and pick the Top Gear West Circuit track.
Start Free Play
1. Set the Max Drivatars to your preference (I set to 15 - Shadow XBL confirms you need at least one other racer)
2. The the Drivatar Difficulty to New Racer
3. Set Laps to 1
4. Set Collision Mode to Always Off
Run each race, then just cycle through your cars. The achievement will pop at the end of the tenth race. You do not have to win the races.