How do I get 3 stars in Mission 39?

To get the 3rd star in mission 39, you have to kill 50 zombies. No matter what I have tried, my best result is 43.

I have tried to use rockets so they hit targets I missed but anyway. It's just not enough. Any tips?

I am playing on an iOS device.

UNKILLED 39


Turns out it is easy after you get bigger gun. The faster you kill then the faster new ones pop up.