Destroy All Humans!

Turnipseed farm, look for a human, damage it so he has half his energy left. Take a hay bale, set it on fire with your zapper and throw it at the damaged human to kill him.

Really Simple, you can get this in Mission 1.
Simply Zapp/Electrocute a Haybell a few times until it catches on fire. After that, grab it with and throw it at an enemy/human.
VIDEO GUIDE:

Here is my 100% Achievement guide and full walkthrough so you don't miss a thing.

Load up Turnipseed Farm free roam.
When you spawn, to the right of you is a hut/house with a furon probe and an explosive barrel.
Also here are a few hay bales.

Now go on a short killing spree until a good few enemies are trying to kill you.
Make sure to use your zap o matic to get the enemies health down to a little bit below half.
Then use your zappo on the hay bale until it catches fire.
Pick it up using and throw it at the half dead enemy for this to unlock.

Seems that some people are unlocking this without having to kill them, so try it before getting their health down to save some seconds.

Go to 28:00 in my part 1 video to see where and how to unlock this

Hope this helps!

Part 1

Part 2

In this video guide I will show you how to set something on fire, then use it to destroy a hapless human. Now this achievement can be done on the first level, Turnipseed Farm. Once you are able to free roam to destroy more humans, you can bring a bale of hay with you. Weaken a human first with your gun the set the bale of hay on fire with your gun. Then use your telekinesis to run the human over with it. Once the bale of hay you set on fire kills the human the achievement will pop.