Broken/The Helicopter Sniping Trick
This trick only applies to Vice City, San Andreas, Liberty City Stories, Vice City Stories and IV. There is a way to destroy a helicopter in one shot with a sniper rifle. When you have an opportunity, if you have a sniper rifle equipped, zoom in on the cockpit window and just fire. The helicopter should lose control and fall to the ground, Good Hunting
The helicopters in GTA IV have increased armor, making them very resistant to small arms (except RPGs), repeatedly firing at the hull will eventually cause it to smoke, but this can be difficult and time consuming. However, the cockpit of a Maverick has large glass panels and the pilot is clearly visible, if you can shoot the pilot with a sniper rifle, the helicopter will be sent into a death spin and crash. This can also be done with the Annihilator, but the front wind shield is much smaller and higher up, making a shot from the ground more difficult, but still possible. In addition, killing the two cops who are shooting at the player from the side doors of the helicopter will cause the helicopter to retreat, spin a bit out of control, or temporarily give up. This will give the player time to outrun the helicopter. It is also possible to shoot the pilot of an Annihilator through the glass at the bottom of the cockpit, although it can take multiple shots since you are only able to hit his feet rather than his body or head.