I posted this in the other Fireworks thread, thought it may help someone here too.
Nothing easier than just dropping a grenade on an enemy below you. I finished the original game long ago and had tried many methods being unlocking it in this manner.
If you can unlock it by just throwing a grenade above an enemies' head on the ground, "great, well done" continue on. If you cannot unlock it that way, try dropping grenades while in vertical cover.