Pick up Requisitioned Fireworks Launchers in Strongarm Airstrip, then use them on 12 Strongarm Gyrocopters.
I spotted some shoulder-mounted rocket launchers on the way over here.
They mighta been fireworks. I dunno. I'm just going to assume that they were shoulder-mounted rocket launchers until I'm proven wrong.
Point is, in the arms of a skilled <class> like you, they could probably take down a chopper. If you find any lying around, try pointing them at those poorly-built gyrocopters.