Well, I happened to finish the mission in the stealthy way, at least mostly.
First, sneak to the left and hide by the statue, tell your pal to stop.
Wait for the guy to get a bit ahead (he walks from the right to the left). Whack him with the bat at full strength.
Go left and towards the house, sneak to the right and to the guy in the booth. Whack him with the bat.
Sneak up to the house on the right side. A guy will be patrolling from the left to the right. When he is walking to the left you can sneak a bit closer. Do this a couple of times so that you're fairly close to where he turns. When he again turns to go to the left run up behind him and whack him with the bat.
The maid has a pretty obvious routine. She is first in the kitchen to the right of the windows, then goes to the windows to look around and turn the lights on. Then turns them off and goes back to the kitchen. When she turns the lights off wait a little bit, then open the door, sneak up behind her and whack her with the bat (no mercy).
On the back of the house a few guys are patrolling. They're annoying, so fill them with led.
The safe you're going for is up the stairs to the left, so find the room. Once you've done that go back for Salvatore (your pal) and bring him to that room and get the safe open.
*MOVIE SEQUENCE*
Now things are a bit tougher. Go outside the room and head to the stairs. Use the shotgun and a pistol to clear out the goons. The guy you're not supposed to kill is down to the right, so don't bother him. Find his car and drive away.
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