Destroy All Humans!

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

Load up 'Turnipseed Farm' free roam.
This does NOT work on random civilians.
Only human enemies that try to kill you work, so get to killing a few humans until enough people are trying to kill you.

There are a few explosive barrels on the map, but one is the hut/house to the right of where you spawn.
When two guys are close together, grab and hold the barrel using , wait til they are close, then release .
That should unlock this achievement.

If you want another one for killing a guy with an explosive.
Get one guys health to below half, then use the barrel.

Go to 26:15 in my part 1 video to see where and how to unlock this

Hope this helps!

Part 1

Part 2

Really Simple. Enter the "Turnipseed Farms" Level in Free-Roam/Challenge Mode. There will be an Explosive Barrel to your right hand side after you spawn. (Also a Furon Probe if you don't have one yet)

Throw this barrel at 2 humans close together for the Achievement
VIDEO GUIDE:

In this video guide I will show you how to destroy multiple humans with a human-made explosive. Now this achievement can be tricky. You can do this on the third level, Santa Modesta. Near the mission objective where you find sleepy Ernst there will be explosive barrels nearby. What you want to do first is start killing civilians, cops, and military men to draw a big group of them. Once you have the cops and military attention, group them up together and place an explosive barrel in the middle of them. Then use the X button to set it off. If you mess up there are more explosive barrels around, or if you really need to play the level in free play.

Mission 03, where you have to kill the scientists. Just shot a green glowing cow and it should kill a couple if scientists around. This unlocks the achievement.