This mission appears after taking out the second Capo (in any order)
Head to the marked location and lure Marcano’s hitmen. This is similar to dealing with Marcano’s hitmen. Make sure to use your Associates to get reinforcements or call off the cops when needed. It will also help if you lead them to one of your allied strongholds. Keep killing them to lure Sanangelo.
Once you’ve killed enough, he will appear with his hit squad to take you out personally. Like before, luring them to an allied stronghold will make things a whole lot easier. After disabling him, subdue him to personally kill him.