To unlock the Chain reaction trophy in World War Z, you will need to hit at least 10 zombies with a single Stun Gun shot.
To use the Stun Gun, you will need to play with the Slasher class as the Stun Gun is part of their equipment. You will be able to get this trophy during the first Episode: New York. Just wait until there are a mass of zombies climbing a fence, wall or lay some barbed wire so that they group up and then fire your Stun Gun using
.
It might take a couple of goes to get it to hit 10 at once.