Press the 12, 13, 14, and 15 buttons on the elevator. Then go up the stairs. The elevator should be slow enough so that if the terrorists are at a lower-than-expected level, they will still be waiting for it when we go up the stairs and take them by surprise.
Follow Tomasque at a respectable distance.
You ascend the stairs swiftly, and quietly, with trained professionalism. There's nobody on the stairs... at least, on the levels you can see. The building is 40 stories tall, and this stairwell seems to go up a fair number of them. From the bottom floor, all you can see at the 20th floor is the underside of the staircase and a faint ray of light from the glass sunroof.
You arrive at the 15th story; this high, beyond where any visitors might stray, it's a simple mechanical door as opposed to the electronic sliding one downstairs. It's slightly ajar; somebody's been through here recently. With an elevator so close by, you can't think of any legitimate reason for an office worker -
Shots ring out from above! Craning upwards, you see three of the office workers-turned-terrorists. Wild and disheveled, there's a feral light in their eyes as they take aim to fire again, leaning over the railing for a shot. You don't know how they managed to bring weapons into a secure office, or why they appear so crazed, but they're not going to miss this time.
I am taking the elevator, might as well provide a distraction in case they are expecting the stairs. Shields up, shotgun ready!
The doors open to a completely normal hallway. However, three shots ring out, and you hear a loud crash as a door is slammed open; a terrorist emerges from an office, pistol in hand, moving towards the stairwell. He somehow doesn't notice you as he passes by, tunnel vision intent on the source of the disturbance.
Red is Dustan, blue is Zormod, green is Tomasque and yellow is the terrorist.