I was having major difficulty with Live Free Drive Hard using the Mclaren (I think!). Many of my speedwall friends had golded it using that car. Best I could do after a 3-hour session was 1.33 seconds outside the required time.
I grinded up to level 30 to unlock the Lamborghini Siesto Elemento, and made the time by more than a second, after about 5 tries.
That car, for me, was the platinum-earner.