Heli Attack 3/Zone 3-3


 * Helicopters to destroy: 8 Bomber Helis
 * Other enemies: 5 respawning, gun-equipped robots

Tactics
Use the same strategy as Zone 2-4, and don't forget the warnings mentioned in Zone 3-2.

Unlockable: Flak Cannon
The third and final secret free weapon is in this zone. To reach this one, drop down twice while you're on the second metal bridge. Destroy the sections of the wall to the right, then walk into the gap you made. When you get to the end destroy the sections of the ceiling above you. Jump up there and you will see a Flakcannon.

Alternatively, this weapon can be unlocked in the Training Zone.