If you encounter this problem this is how you fix it. I noticed the achievement was being reset to the kills made in the last race of the tournament, once I knew this I just killed over 10 in the 6th race and although the achievement never actually popped it reset to above 10 and that was that. Now all I need is that truck and I think you only need silver to get it. Good luck it is a great game.
I have done this achievement so many times while playing to earn the truck yet never get the achievement. This can be seen on the Leaderboard under Driver Tournament. Event 3/6.. Position 5014 Mykill100 with 15 kills on that blood race. I have passed 10 around 8 times yet the achievement will not register the kills. I have tried restarting the game because the percentage changes everytime and after my last attempt at this achievement it reduced to 33%. It's not that difficult to kill 10 cars but impossible for me to get this achievement.