World War Z

To get this achievement you will need to play with the Slasher class as it starts with a Stun Gun. It is recommended to do this offline to prevent other players from killing the zombies you want to target (Or try to do this with a session only with friends if possible).

Finally, after choosing the Slasher just give a single shot that reaches 10 zombies (To shoot with the Stun Gun you must press the LB button). I got mine in NY in Chapter 1 shortly before reaching the train driver (there is a small horde attacking the main train wagon before you talk to the train driver). My friend got his in NY in Chapter 1 also but he got when the zombies begin to climb the first big wall while we are still in the Mall (shortly after descending the escalators). In some attempts you should get it.
Good luck!

What i did with an unleveled Slasher:

-Play online - private
-Easy difficulty
-New York: chapter 1
-Dont fill empty slots with bots (game settings after you choose map)

Go down the first set of stairs and shoot with your primary to get the zombies attention.
Run back up and try to move around (or shoot a few if the herd is too thin) until a group of zombies are around you.
Press LB to fire the stun gun into the crowd.

Doing it this way i got it first try. I did try 2 times before not running back up that failed.

It might be a bit zombie spawn dependant, please comment on how it worked for you, if it took several tries etc.

In order to hit 10 zombies with tazer shot you need a stun gun which is part of the Slasher Class equipment.

I got this achievement in Episode 1: New York, Chapter 1: Decent. In the mall section before you enter the subway at the final horde battle. I was playing offline with bots. I planted a barbwire fence waited until the zombies grouped up on the barbwire and fired.

This worked my first time trying it this way. I was finding it hard to hit 10 zombies when the hordes were directly running at me.

This is a video guide to unlock the chain reaction achievement for world war Z.