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This may make me sound like an idiot, but for anyone out there who was trying to get the achievement by loading the mission at the hijack checkpoint...wrong. You have to load from "chase" and then it is incredibly easy. I came out with around 3 minutes to get through everything once I actually loaded the proper checkpoint. Basically just do what the achievement says from the proper checkpoint and you will have no trouble. You can get this on easy so you shouldn't have any trouble.

-A Dr. Joystickler Video-
This is just a video with a few tips to make it easier. It works just like SHiZ NL mentioned above. This achievement can be done on any difficulty. The only hard part is if you die you must restart the mission.

Easy achievement. When the mission starts you will be in the helicopter you stole at the end of mission 5. Now all you have to do is shoot all the enemies on the road. They are very easy to kill with the rockets on the helicopter. (Press left trigger for rocket, and press the right trigger for the machine gun.)

When you reach the convoy of tankers shoot the hostile vehicles (3 of them) who escord the convoy. Aprox 3 to 4 minutes after you started the mission.

Now comes de easy part, take over the trucks. Simply slow you speed and decent (RB is up, LB is down) to fly beside the first truck (last in line) and press "X", repeat this for the two other trucks to stop the timer.

The easy part is, that the helicopter can't be damage when you hit the ground or the trucks. So you can "snap" to the ground and a truck without damaging the helicopter. This make it very easy to complete this task under 8 minutes.

Very easy achievement and not longer to find it, just see this video to know how to do this without damage. Just press X when you are at the truck and achievement go after 3 ! Good luck