At the start of the level where you reach the security guard, find a chair. It is easily moved. Move it to the door that he will open for you, and get him to open it. Slide the chair in between the doors, and it will not close. This might take several tries. Go out, take the crowbar, charge it up until you reach the same security guard, and hit him until he explodes. After that, get his pistol with the laser sight and continue the game.