This one is missable, and i almost missed it. When you have an opponent to stop, you need to completely destroy their car instead of merely stopping them.
I found the easiest way was to nitro in to the back of them from a distance, or push them at high speed in to a wall or a truck.
As soon as you see them smoking from the back, one small shunt will finish them off.
Start on this one early to make sure you get it.