Mission 64 - Steal Frankie's Blackmail Stash
Skip the lobby and go in through the back, southwest corner. Staying low, take the north stairs up. You'll see a lot of guards about, but they're not very alert.
Make your way to the target floor, and creep against the wall to a room with two waiting guards. You can try to stealth them, but either way be ready for a fight- inside to the left is actually a huge mass of goons, all standing together at the bar to look like a single guy on the minimap. They rush you at the first sign of trouble.
Gun them down. Prioritize the sentry, but kill active shooters first. The phone is right on the desk, and the sentry will be a motionless target for many long seconds as he dials and calls. Watch your back for a lone guard in the small room behind you, then loot the hell out of these rooms- cash, collectibles, and the blackmail evidence you're looking for.
Collectible alert: Vargas painting in the office.
The guards are likely on alert now, but they travel one by one, easy to pick off. If the lobby level is too crowded to exit the way you came, make for the northwest corner, into the laundry. There's a medkit here, and a stashed Carter M33-A, for some reason.
Head down the stairs, grab the fuse lying here, then hop a ladder up and out onto the street.