Vengeance

You begin by needing to take out the guy on the right first. Then, turn your attention to the dude at the front desk. Grab the shotgun he drops and head into the next room.

This next area is where you can really control the flow. Hang back in the room you just entered and let the enemies come to you. Shoot the first guy that rushes in, and then focus on the other two who will follow. Keep yourself positioned here and continue to take down any enemies that get close. If you run low on ammo or need a different weapon, grab one from a fallen enemy. Keep this up until only a few sentries are left. At this point, you'll need to sneak up on them to finish clearing the stage. Once that's done, grab a loaded gun and head up the stairs.

As soon as you're upstairs, some patrol dogs will spot you. Aim towards the north-east and start firing immediately. Your priority is to kill the first few dogs that rush you to calm the situation down.

Next, you'll need to clear out the west hall. Use a melee weapon for the regular guys in the hall, but switch to a gun for the heavy enemy and the sentry. Warning: Do NOT enter that room directly. The enemies on the other side of the glass will shoot you through it. Instead, equip a gun and take them out from a safe distance through the windows.

Now, move to the east hall and clear out any remaining enemies. Make sure to get anyone still inside the room just before the boss's office. Once that's done, enter the office to get your next objective.

Keep an eye out for a Puzzle Piece. It's located in the bathroom, which is across from the entrance on the second floor. Make sure to grab it!

Finally, head back to the car to finish the level.