Mission 101 - Kill 'Handsome' Harry's Enforcers
Come in from the northwest. Your shooting position is the top of the stairs.
Use something silenced to take out the balcony sentry, then the near guards below, then the other guards that come to investigate the near guards. If you do this without raising the alarm, Irving will remain... distracted, and you can either blow him away or take him down at your leisure.
He's got a Viper rifle and a ton of cash in his room, all of which can be yours.
There's a front door and a basement door, but either way there's couple of guys just loitering annoyingly between you and your mark. I actually recommend the front door for this one- you'll come up behind Ernie for the kill. Just follow your objective marker upstairs, killing any hostiles in you path. Don't forget the targets of opportunity on the lower level.
Come up from the southern alley. Use the doorway here to lure over the nearest guard, then shoot anyone who remains, including Oscar up on his balcony. Search his room for a bag of cash, and get gone.