Quite easy, start before GOAT Z and install 3 blue barrels (with smile butterfly) on the camping car.
This camping car is near UFO crash site (when you face UFO, there're 2 paths on your left, take the 1st one, the second is the church path).
1 blue barrel is near the camping car.
The second on the 1st floor, in front of UFO (behind you when you face UFO).
The last is on the roof of the mall, near the pinata.
When the camping car is set, it blows cristals on the map. Now you can infect people.
It's a time limit weapon, you can't freeze n destroy 30 zombies in one run.