I got this on the first world, Desert Strike, level 14, using the Detonator bought from the store for 160MSP. FWIW, this ability can make some difficult levels a lot easier!
Anyway, on level 14, there are three guys on a bridge, lob a grenade so it hits the middle guy on the head and detonate the grenade.
I tried doing it without the detonator, but with no luck, but it looks like others have managed to do so. See the comments.