Hella Copter

From HelpMeWithMyGameProblems.com

If Frank manages to shoot down a helicopter after the Special Forces arrive, you'll earn the Hella Copter achievement.

The consensus seems to be that the best place to shoot down a helicopter is from the roof of the covered picnic tables in Leisure Park. To reach the roof, perform a Zombie Ride on a zombie that's standing near the edge of the roof, then leap from it's shoulders toward the roof. Frank will either land on the roof, or will catch the edge and pull himself up if you've executed the manuever correctly.

Try to hit either the main or the tail rotor on the helicopter. Expect to take plenty of damage, and fire plenty of shots before you succeed.