A bit before halfway through level 4, "Travel To The Airport", you will go through a narrow tunnel on your right. Proceed until you see a SHTURM (missile launcher) near a building. Defeat it and continue left before the building. Shoot the watchtowers from a distance. When you get close, the man floats in the air even though the tower he stands on has been destroyed. He can still shoot -- attack him with a grenade.