Taken from my guide here: http://www.xbox360achievements.org/forum/showthread.php?t=20...
Level Walkthrough – Objective by Objective (on agent)
Objective 1 – Gain Access to the Laboratory.
All you need to do is get down to the ground floor and find the elevator leading to the underground lab.
Once the mission begins, jump down off the helipad and head down to the roof door. Shoot the guard here if he sees you, otherwise knock him out from behind. Head into the building, then as you reach the top of the ramp leading down; there'll be a guard right around the corner so get ready to shoot him. Head down to the next door and head down the stairs. You'll now be on the same floor as Cassandra's office. As soon as you enter the floor, call the elevator to your left and wait for it to appear. Jump in and head down to the ground floor.
Once there, leave the elevator and go down the stairs. Go left and head through the door there. A few guards in the next smaller room will greet you, kill them then go right around the corner and part of the wall in front of you will open. Go in and activate the elevator on the right to finish the mission.
Mission Complete and Achievement Unlocked! Well done!