When you head out on your way to the maintenance tunnel you will run through a giant outdoor area filled with zombies, lawnmowers and a fountain/lake thing.
After a certain amount of time three convicts will appear in a jeep with baseball bats and a turret and beat down an old man.
Simply take out the guy on the turret with whatever guns you have by luring him into a tree - the AI will stumble and keep reversing and driving back into the tree so you can take them out easier.
Once the turrets been taken out go and grab it off the back of the car and use it to defeat the other two convicts - killing the driver before the passenger will cause the passenger to take his place driving just for the record.
Once they all have been killed simply jump into their jeep and the achievement is yours!
-- OR you could try the method in this video below.