Plenty of heavily armoured guards lurk through this mission, so stealth is key. In the left corner there's $7500 to be had. Behind the briefcase you can cut power to the building's surveillance system.
Cutting the power will alert a guard to come downstairs. Ensure you're hidden behind the shelf in the middle, as the guard approaches from the left hand side. Take him down and move to the stairs he's just come from. Go through the door and sneak to the bottom of the stairs. A guard patrols above, so let him stroll past and quickly take him out.
Round up all the items on this level and go to the computer room on the left. Deactivate the camera system and then hack the next computer to complete an objective.
Stay by the wall on the next flight of stairs, and wait for the guard to walk to the left. As soon as he moves take him out. Enter the next room behind the stairs and hack the computer. If you get this wrong you'll trigger an alarm. As the code is smaller, it's best to quite out of the hacking action rather than making a mistake if you think you're not going to make it in time.
If you have to kill any American Agents, Mina will begin to dislike you. Each time you do it'll be -1 from your reputation with her, so triggering an alarm can have monumental effects. If they spot you, get in close and beat them down to the floor.
There's a locked door next to the computer you've just hacked, so get through and pick up a bunch of helpful extras. Head across the hallway and into the next room. Pick up items and hack the computer in there, before modifying the air conditioning unit.
Take cover by the side of the air conditioning and wait for the agents to sprint out. You must deal with them now. Follow the guard who runs directly past you into the next room, and take him out before he enters the doorway to the landing. Get rid of the next guard as he stops just outside the smoggy room.
When they're gone, get inside the room and hack the computer. There's ammo on the coffee table and a briefcase on the side next to the computer.
Now descend the stairs so that you're looking at the front door on the first level. A guard will appear and you need to take him down quietly, so utilise a tranquilizer if you have it.
Go through the front door to complete the mission.
