Before starting this mission at the phone in the airport terminal, find a helicopter and park it outside the airport. Also, get body armor (preferably with 150 armor or health). Start the mission, get the gun, and go to the farthest metal detector from you and snipe from there. Then, enable the "Lower wanted meter" code and run to get the briefcase. After that, run out, get in the helicopter, and fly it to the Downtown Ammu-Nation store.