The 4th New York stage has some turrents right there at the start of the stage, about 60 seconds in if you just run there. There are some on the top level and bottom level. I think their spawn points change just a little every time, but they are always there abouts. Just put the game on easy, private, New York, 4th stage. Run up to the turret and you will see the option to take convert it. Just do that to the few turrets there, quit out, and repeat.