This.
Hell if your dealing with slow zombies, all you need is:
- Two lines of spearman
- A few "spotters" whose job it is to pull any of their buddies out of the fire that get tripped up, or just in case of any shenanigan's. They are also watching for zombies in unexpected places.
- A few supply people, who ensure their spearman always have spears. Obviously some will get lost or broken as the fight goes on.
First line stabs the zombies and steps back. Second line stabs the zombies and steps back. Rinse and Repeat. As long as you have space you can basically kill infinite zombies. You can do it with a single line, but two lines gives you a bit more coverage and lets each line work at a leisurely pace so they don't fatigue too quickly.
Just start outside a populated area, blow the horn to get the zombies to come to you in a nice open field, and just begin the work. It might take a while but you will easily clear out an area of hundreds of zombies in short order.
As cool as slow zombies can look....there's no way they take out the world. At least in the World War Z movie, the zombies are so crazy fast and do that crazy group up thing that at least makes them a credible threat.