This mission is rather easy.
Is you stand in the alley across the street to your right from Luigi's, they won't notice you. Then take out your sniperrifle and pop Salvatore one in the head
Or you can run up to his house, and block the entrace with a coach (and a few cars)
wait for him to show up (and get with in explosion range of the coach) and nuke the coach and surrounding cars, hopefully setting his car on fire, or blowing it up.
If he bails from teh car, run him down with another car
)
