When you're on the equipment selection screen choose to have a breaching charge. If you can get to the two doors that lead you into the library second floor.
Crouch down in front of both doors and place a breaching charge on any door.
Whilst still in front of the doors repeatedly press the A button, this will open and close the door.
Keep doing this for as long as you like.
When you have had enough blow the breaching charge (this will not kill you) and have a look and there will be many bodies of terrorists on the floor and a mass increase in terrorists left to kill.