This is the easy way to kill the convicts in the Jeep at any level.
At the begining of the game you can go to the gun shop and get a sniper rifle without having to deal with the crazy guy that is in it... or after you beat him you can go get any gun in there.
But after you get the sniper rifle (I would get two) go to the park at night when the convicts are out running around in there jeep with the 50. cal mounted on it.
All you have to do is run around alot until the driver drives into a tree... I would suggest killing the guy on the 50. cal first so you dont have to deal with getting shot.
But just run around until they hit a tree and it will take them a while to back up and start going again so while there going back and forth just continuously shoot them in the head.
After they get unstuck run around again until they hit another tree and repeat the process.