To get this first you have to get 100 Hot Wheels medals then you are told about the final race which is another Golaith event which you have to win. It is one lap default and it took me under 7 minutes in a stock Pagani Zonda R. You can also get the Hot Lap achievement if you manage to complete the race in under 6 and a half minutes. (Maka91 has a great guide)
Added from Spilner.
A note for you, a friend and I did this in co-op and the achievement didnt pop for either of us, replaying in single player however popped it.
Added from Los Angeles PD.
the 6mins and 30 seconds achievement is incorrect i did it in 6.20 and did not receive a achievement.
I'm not sure about this as ive not got it yet so if anyone else has this issue and knows how to fix it please comment and I'll add it. Thanks
If you have anything to add please leave it in the comments. Thanks.