This mission opens up after finishing The Bus Assassination and can be found at the "L" marker on your map, located in La Mesa. Answer the phone in front of the liquor store to start the mission. Lester wants you to kill Enzo Bonelli, who went from being in the Mafia and is now in real estate.
The only problem is that this assassination won't be as easy as the others, as Enzo knows there is a hit on him. That means you will need to bring some firepower with you, and it might be a good idea to get some armor as well. Grab a car nearby and then head to the construction site where Enzo is located.
When you get there, head inside the construction area and fight your way to the elevator. Go up and get ready for some more enemies to appear as soon as the elevator stops. Hunt around the area for the remaining enemies and then head towards the next elevator.
The elevator comes down with two men inside, so quickly take them out and ride this one to the top. Once you get there, get ready with a RPG (hopefully you have one) and then switch to another gun. Kill a few men and before the helicopter can take off, blow it up with your RPG. Take out the remaining men and grab the parachute from a nearby workbench. Jump off the construction site and leave the area to finish the mission.