I got this achievement in the red zone. Simply steal a tank from one of the military bases, and destroy the other ones that are in there.
At this point, you should already have 2 kills. If you do it at the base in the top-right of the zone, there are 4 helicopters behind the building, putting you at 6 kills. Just drive around a bit, destroy the strike teams if they are called, and look out for any other tanks that you can one-shot.
Even on hard difficulty, this isn't really that hard to get.