2-1: In Case of Fire
You begin this level in the elevator - you get a CS showing the enemy nearby, and then you get to kill them! A nice explosion gives this floor the feeling of an open-air atrium, and you get to fight your way through the desk farm to the other side, where at one point a body comes raining down. There are plenty of enemy to kill in close quarters, and eventually you work your way to a set of stairs that lead down to a metal grate you have to co-op lift. On the other side you go around a corner and get the Mock Surrender tut - go ahead and follow the instructions here.
Collect 4th Radio
When you do this you get a bullet-time effect and the pleasure of some nice slo-mo head-shots! Once they are dead, note the Radio (Radio 4 of 14) on the left desk - go ahead and grab that now!