There are 5 cars in the Movie Legends Pack, and each car has 6 events. Beat the gold medal score for all these events to unlock this achievement.
If you also have the Terminal Velocity Pack, you'll see 2 extra events (a Drift Attack and Smash & Grab event) which are not required for this achievement.
These are the cars:
Aston Martin DBS 2008
Aston Martin DB5
Pontiac Firebird Trans-Am Special Edition 1977
Dodge Charger R/T 1970
Shelby GT500 1967
The first 4 cars are available as soon as you've purchased the DLC. The Shelby GT500 is earned by winning the new Shelby GT500 Most Wanted event.