Just to share my experience with this. I finished my hardcore playthrough and have been doing my 2-4 runs to build tech points for the grease monkey achievement. During which, I was attempting shooting them with the magnetic gun and then blowing them up while they were in the air. I had already blown up way more than 10 guys during my campaign, so I figured I was close. At any rate, I ended up getting it when I blew up a standing, definitely not airborne, bad guy. No idea....
I prefer to keep a minimal friends list, so if I add you as a friend for the purpose of boosting a game and remove you later, I assure you it isn't a personal thing.
*100,000 hit on 5/17/10, only took 3 years and 144 games...*