*First ever guide so bare with*
Really simple this one so get into a later level. Say 20 etc.
Get the helicopter gunner from the crafting table and start a new level on the wing.
Make sure you have upgraded your gun as much as possible. I used the shotgun. Think I had packed it twice. Stay on the wing until you start to get overrun with LOADS of zombies shooting as many as you can.
Enter the helicopter and then fire away. This is how I popped it.