My experience in this is that I played the entire campaign twice, once for the hard difficulty cheev and once for journals on casual, and during both I always aimed to get airborn kills with grenades. Then during mop-up sessions I did the same. Finally, I played 1-5 a few times killing the Knights with grenades only to no avail, so I tried 3-3, I shit you not, the first grenade I threw nets me the achievement. I think for those aiming for this one, 3-3 is a better choice.
I think in total I would have well over 50 airborn grenade kills but who knows how this one is supposed to work
The pain, the torment and torture, profanity, nausea, suffering, perversion, calamity. You can't get away.