You say it is not possible but in all reality it is.
All you have to do is some map editing put box's in certain spots where they show up for you and then put crouch down, When they shoot at you it won't hit you when you look up and shoot at them it hits them, however it would be a good thing, but it would take some time to find out how to do it, What maps to do it on. ext ext. See what I mean. Hell I seen a dude that had stairs going up to the bridge on kill creek, the middle of the bridge he was walking on thin air wasn't a fly hack just walking on thin air, so it is possible, just would take some time to figure out how to do it.
Did you ever think he had a hover hack? It wasn't invisible stairs.