Just follow these steps

1.Build a machine gun at the exit nothing else.

2.Build a second machine gun when you have the money and also build
  a healing weapon.

3.Turn both machine guns to gatling guns and start building a third
  gatling gun.

4.When you have built at least 13 of these (make sure they are blocking
  the exit) start building chain lightnings more apple trees and healing
  weapons built near or as close as possible to the exit.

5.I lost at level 50 because they get 100000 hp there so save some money
  and then build tons of nuclear barrels to kill them do not worry but 
  your armada will lose at least 20 weapons. Thank you for listening.