Hella Copter To obtain this achievement you need to get to day thee of 72 hours mode, if you have not made it to 72 hour mode and have not finished the game do not read this solution as it contains spoilers.
*** Spoiler - click to reveal ***Grab a bunch of machine guns from the Special Ops and some good mixed drinks and head outside into leisure park. You are going to want to jump on top of the gazibo overlooking the picnic table, you will want to memorize the pattern the helicopter goes and shoot slightly above it.
Video is done by youtube user gamerfourlife99