Just thought I'd add a guide as it took me 5 minutes to figure out what this Achievement was specifically for.
Firstly you need to enter the touring car category and complete 10 events. This doesn't have to be wins just has to be completed.
Then once you've completed ten events the showdown hammerhead will unlock and appear at the end of the touring car category.
You then need to win the event and the achievement is then yours.
You can change the difficulty as well to make this easier on yourself.