Mission 34 - Confront Roy Thibault
This is a return to the same warehouse where the medical truck was stored, and similar tactics apply. Whistle guys to the door, and take 'em out. When the path is clear, climb the stairs to the near sniper perch.
Immediately after the sniper is down, crouch by the door and whistle in the guy down the far stairs, before he spots you on his own. Now turn back to the east door again and whistle up the sentry there.
Once he's down, you have breathing room for either sniping from the perch, or continued stealth. If you do plan to start firing, consider also the sniper perch in the middle of the space, directly above Roy's office. Either way, you might still want the stealth for a bit, just thin out the numbers a bit.
Assuming you do, go down the east stairs, advance to the desk, and whistle up the sentry here.
Proceed through these small rooms, then whistle-lure the vulnerable again. Be careful to note who actually responds- there are a lot of hostiles nearby, and sometimes you'll catch a guy from the upper floors. From here you should be able to take a sizable chunk out of the building's population.
Once this area is cleared a bit, sneak past the sniper point, to the small rooms on the other side where you can pick up a tac vest, and then a medical kit , hopefully taking out the last sentry here as well.
Lastly, creep back to the stairs, take out the sniper, and then just shoot whatever small handful of enemies might remain. Roy remains in his office no matter what, so resistance should be minimal. When the floor is clear, walk over to office and shoot Roy in the face until you become best friends.