Once you have killed all the enemies inside the bar, you will need to exit and kill the enemy that flees in the car. His car is in the back I'd the bar and that's where he'll take off from. You can either exit the back of the bar or the front where you initially came in. I would suggest leaving through the front as it will be quicker and easier way to get the kill however, going through the back is not difficult. The only difference is if you exit through the back the enemy will take off and you will need to jump in a car and chase him down until you can immobilize his car and kill him whereas if you exit through the front you will be ahead of him allowing you to shoot him right in front of the bar after he takes off. As long as you can shoot him before he drives past you, you won't need to chase him down. If you're walking out of the bar he will be coming from the right so just be ready to fire. Both ways are easy to do, that way is just a little faster though. The video I posted shows when I unlocked the achievement.