This can be done very early on in the game as long as you have a few firecrackers. There are a ton of locations that this can be done, although an easy one can be seen below:
The easiest way to get a group of zombies to be simultaneously shocked with electricity is to make them gather in a puddle of water next to an electrical switch. Jump on top of a building or dumpster and use firecrackers to get the attention of the zombies in the area. Throw the firecrackers into the pool of water in order to make them stand inside. Once 5 of them are standing there, jump down, interact with the switch, and fry them! Achievement unlocked.