In the exchange level at the end when your fighting the helicopter; first shoot out 
the windows or else when u fire the sentinel you'll be killed; then take out your 
sentinel and fire near the helicopter use the up left and right joystick to control 
it left to right and the c button from top to bottom (this also works for mini 
helicopters in multiplayer). Shoot the helicopter 3 times and your done 
Submitted by gtb365