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Helicopters to destroy: 12 Manless Helis
3 respawning Gunned Robots

[edit] Tactics

Use a weapon to take out two gunned robots then stay in that area so you won' be hassled by the robots again because as long as they don't go off the screen they won't respawn. They can only fire in straight lines so if you crouch they may not be ale to attack you, this also allows you to attack them from the air without taking any damage.