To get to level 50, build a machine gun at the exit nothing else.Then, build a second machine gun when you have the money and build a healing weapon. Turn both machine guns to Gatling guns and start building a third Gatling gun. Make sure they are blocking the exit. After building at least thirteen of these, start building more apple trees, chain lightnings, and healing weapons built near or as close as possible to the exit. Save some money to build lots of nuclear barrels.