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