Heres another tip if you dont mind waiting some time, as the previous poster mentioned get into the truck, but once you are in the truck
1. You need to turn on infinite time in the cheats menu (within the pause menu)
2. You need to go right and find the gun turrett, then drop it on the truck
3. Tape your
button so the gun shoots by itself, saves you holding it down
4. Flip your controller over and put it on the ground so the stick faces up due to the green slimy looking guys wont get hit if the stick isnt up
5. Go grab a snack or something and 15-30 minutes you have your achievement
6. Make sure to move the controller before 15 min is up as the controller will shut down or just plug it in if you have the USB cable
Also works for this one