For anyone not sure what to do: you have to learn how to time your clicks on the control buttons on the right so that Kong's fist hits the choppers or people just before they collide with his head. You have up to 16 swings of each fist before Kong gets too tired to punch any more. The top two buttons become active when he gets close to the top of the building.
Excellent level! Quite irritating choppers. 5/5 from me. But why won't the choppers stop moving when I attach the chopper's freezing node to one of the freezing nodes in the tower? They just keep on flying like nothing's happened.