Get a sniper rifle from the gun shop before the owner appears and you must fight him. If you do not make it and you must fight him, the easiest way to take him out is to run back out of his store, kill the zombies outside, then shoot him with the handgun or Uzi. When you get the rifle, go to the park where the three prison convicts are found. There is a picnic area with benches just outside and to the right of Paradise Plaza. You can jump on top of this structure. This will keep you from having to worry about zombies jumping on your back while you scope the jeep from far away. The jeep usually seems to get stuck near the food court with its back to you. While on top of this structure, you can actually kill all three convicts with the rifle if you are positioned correctly. Once they are dead, just jump down to get access the .50 cal.