Manny is pissed because he looks gay in the video the camera guy has been recording. He wants you to shut up some people who have been dissing him. They're arriving on the El Train.
Follow That Train!
Grab a vehicle and make your way to the Algonquin Bridge. Once you're there, park in the mission objective triangle and watch the short cutscene. The guys you need to kill are on the train and it's your job to follow the train, and mow them down once they get off. Follow the tracks by road until the train comes to a stop. You can use the chase cam to see where the train it going, or use your map which will keep track of the train on your GPS.
Whatever you do, don't lose the train. It's on tracks and it can only go one way, but if you get too far away, it's a mission failure.
It'll stop eventually about three stops later and the guys will get off at the station. You can either head up to meet them, which I recommend, or wait for them. You'll notice some parked cars under one of the platforms. They are most likely the ones belonging to these guys, so make sure you stay near them to stop them escaping. Once they spot you they'll talk about how they don't like people giving them the eyeball and then start shooting at you. Kill all of them. Use a drive-by, or get out of your car and pop them on foot. Once they're dead, you may need to escape any cops who heard the gun fight.
- Figure out which set of steps the gang are going to come down and wait at the bottom, behind some cover for some easy pickings.