This mission appears only in Hard mode.
First off, there are some new controls here. 
, 
, 
, and 
all fire your weapon, but it works like the 
: fires left, + fires up/left, fires up, etc.
Your helicopter cannot be damaged. You only need to worry about your character being hit, but this can be tricky enough, especially if you don't have a lot of health left after beating the T-Rex. (It really helps to have a continue here, because you will restart this mission with a full health bar.)
The helicopter has several components that need to be destroyed:
- The missile launcher
- The chaingun
- The bomb hatch
- The thing to the right of the bomb hatch that protects it
- An area next to the missile launcher that has no apparent purpose but must be destroyed anyway.
Destroying the missile launcher is your number one priority, because it is by far the biggest threat to your survival. After that, take out the Biosyn guards in the windows if you didn't get them already, then destroy the rest of the components as you get the chance.
When the helicopter is passing from right to left at the top of the screen, the only way to hit the missile launcher is by aiming diagonally. Stay in place while you wait for their helicopter in position, then when your bullets make contact with the launcher, press and hold right to keep a steady stream of bullets going to the missile launcher. Follow the same procedure when the helicopter passes from right to left at the bottom of the screen. You can shoot out the missiles themselves, too, which you should do when the launcher goes out of reach.
Avoiding bombs is mostly a matter of staying out of the way of the bomb hatch, but be careful to dodge them when the helicopter passes overhead very quickly. Remember, the bombs can only hurt you if they hit your character; they cannot damage the helicopter.
When the helicopter passes from top to bottom or bottom to top, firing the chaingun, try to aim diagonally at the chaingun and follow it up or down. Also note that the chaingun is always easier to hit diagonally than horizontally, due to its orientation.
Once the helicopter goes down, you've beaten the game!