It's called walling. It's fairly known around the game.
I do it all the time.lol
The feet glitch just a lil through the floor,
just enough for the bullet to hit them.
and if you have a 1 hit sniper. well their dead.
also most Cement walls on wave can be penetrated too.
if there right next to them.
oh yeah, in 2towers if your on the top floor.
you can shoot down at the unsuspecting for head shots below.
if your going to wall some one. use a 1hit sniper