Place another one on a surface near you and walk through. Keep your cube facing the one across from the entrance while pushing the other ones down from the side. Turn around again and do the same with the turret in the next room. Quickly turn around and run to the other turret, using the cube as a shield, and push it over.
Run into the hallway with the two turrets facing each other and knock down the one closest to you. Once you are behind it, push it down the stairs.
Knock it over if you want to be spiteful. It is just close enough that you can run behind it without getting shot.
Once GLaDOS is done talking, pick up the turret in front of you and toss it to the side.
