The method I used was a complete accident. I threw one of the gang members that I killed off a mountain and he landed in the forested region below, effectively getting his body stuck in a tree.
I ran down the mountain (which took me about about 60 seconds so that I could kill more people on my decent) to finish off more of his gang member buddies and threw a grenade in the same area that the body had landed. The resulting explosion knocked the body out of the tree and the achievement popped.
So, what I've learned, and I hope that you can to, is that the body only has to remain in the air for 10 seconds, you don't have to keep it in the air for 10 seconds. (That, and apparently trees are of the element AIR rather than EARTH)