I had a simular problem with one of the new DLC cars. I had to take him down 5 times before it gave me credit. So, try this, ( I know its frustrating) take your best car. Beat him, then keep doing take downs till it unlocks. It took me 5 times, but I did finily get the credit.
IF you lose the boss car, just drive around and wait for it. It will come back. Don't enter any other race, just keep driving around.
