Copied from a post back in the CTA because this still exists:
Sandstorm says that you have to throw back an enemy's live grenade and get a kill in order to get the achievement. I, however, got the achievement by:
F*cking up a grenade
Picking up my own grenade
Lobbing it at a bunch of hostiles
I got at least one kill and got the achievement. Either the description needs to be updated or the achievement needs to be modified so it only triggers on enemy grenades.