this was the only achivement to give me problems with this game. Grinding 1-5 a few times worked, but the achivement unlocked when a grenade bounced off the final tank of the stage and killed a random ground enemy.
killed way more than 10 airborne enemies with grenades (not including ones suspended by gravitar or blown up in vertical cover sections)
screwy achivement is screwy.
Also, not sure if this helps or not, but at the very end of the village section of 1-5 (you'll know where it is, it ends at the base of the pyramid monument) there are 3 enemies in the very middle of it with jetpacks that fly between the left and right side. You can spam grenades straight up and nail them as they fly across. If you run out of nades, there's a toy chest nearby that should be listed on your radar where you can get more.
gamer tag: Archer Spense
no random friend requests accepted.