Ok I'm not sure what boss this is but the one boss that is close to a bridge chase him there and shoot him in his head then collect all the orbs and do the same thing again except shoot him twice then after that double that and keep on doing the same thing (this might take a few tries)
STREET RACING ACHIEVEMENT EASY!!!!!!!!!!!!!!!!!!!!Added 25 Jun 2008, ID #2691
On some of the levels,you'll pass by a passage way to the keep were You can trade of your old car with the SuperCar!(you can only do it On the first lap)Once you do that you can either zoom past them and Win, or , you can kill all of the racers and win by default. HAPPY RACING.
When you take out the Los Muertos kingpin, if you take his dead body on the bridge and then crouch a few times, he should COME BACK TO LIFE! I mean, all his bars of life will come back and if nobody is shooting at you, he will get up when you're holding him, get off you, and stand up on the bridge. He won't fight or move but it's freaky, and if I remember right, each time you kill him, you get points toward whatever you kill him with. I killed him about 20 times in a row!