I'm not having any luck with this method. I'm at the scene where you first get grenades on Crashsite and I've made sure that the grenade is cooked and detonates in midair and even directly above the enemy's heads. Not including the ones that were beneath head-level, I have killed about 63 enemies so far and I've had no luck in getting this. Does anybody else have a trigger of some sort? Something they did immediately after they tried this that caused it to pop?
Fear not, my brothers! The next generation of gaming is upon us! Be glad! A reward for all your toil and all your sacrifices is upon us.
There are those who said this day would never come. What have they to say now?
(Yes, I ripped off a quote from Halo 2 and modified it to my own purposes. Sue me.)