This is a Gravina Crime Family mission.
"Sweet Federico is Big Sal's accountant and the only man who understands the Mob finances. He runs a healthy sideline blackmailing fellow gangsters when their books don't balance. Take him out before finishing Gravina, just to make sure none of these dumb fuck guidos can pick up the pieces."
From the start go through the chain link gate into the alley and head left. If you're quick you can look between the two trucks parked there and be able to see Federico by a limo making his retreat further into the alley. He's the one with the large green arrow above his head. If he's there you can quickly fire off a shot and end this now; if not you'll have to get him further down the alley.
If you got lucky and killed him from the start then only the few gangsters immediately ahead of you will remain. In that case finish them off and head out of the alley to the safe zone in front of the Empire General Hospital to end the mission.
If you didn't get him then make your way down the alley, keeping in cover and killing off the gangsters as you go. They will only spawn ahead of you, but there's a high probability the police will get involved and come up the alley from behind so keep an eye on your mini-map and take them out as necessary. There's a lot of vehicles parked along the way that they use as cover so make use of those by blowing them up when you can.
If you don't get Federico by the time you reach the end of the alley, he will get in a limo and drive off. If this happens try to shoot the gas tank, otherwise you'll have to hop in a car and give chase. He will drive fast, but he's pretty easy to stop: just pull ahead of him and force him off the road. When he stops he will get out so kill him now and get it over with once and for all. When he's dead drive to the safe zone to end the mission.
- If Federico drives off at the end of the alley you may just want to restart the mission. If you went that far without getting him you most likely have a four star wanted level by then and any police that see you chasing him will immediately begin pursuit, making things very difficult. On the other hand it presents an interesting challenge so it's up to you; there's plenty of time on the clock for this mission so no need to worry about that.