The following is a walkthrough for Keep Your Friends Close... in GTA Vice City.
Overall, not difficult as you can get well-armed up before you trigger the mission.
The mission begins in your office at the top of the main staircase. Here, your duty is to stay put and slaughter incoming enemies as they try to access your office safe. Several of Sonny's goons will attempt entry to the office, and it is your job to kill them before they get inside. A Colt Python spawns in the office at the onset of this part of the mission, making this process easier. If you need to, step outside of your office to eliminate the men. Once about 20 men are down, Lance Vance emerges from a hallway with many goons protecting him and teases Tommy to follow him.
Now it's time to kill Lance. He will enter on the second floor of the main hall, overlooking the main staircase and office door, wielding a Kruger assault rifle. You and he will engage in a short conversation before he begins shooting at you. At this point, you need to chase Lance down the hallway leading to the interior staircase while he and numerous Forelli goons take shots at you. Upon reaching the interior staircase, Lance will head for the roof.
Crouch behind the barrels on the roof for a small amount of cover. Using the holes between the barrels, shoot Lance and his bodyguards. Once Lance's health is reduced to halfway, Tommy will yell, "This is the last dance for Lance Vance!" Lance replies, "I said, I had enough of that at school!" Once he is completely dead, Tommy will yell, "You picked the wrong side, Lance..."
On the roof, you also have the option of taking out the Forellis' limos, ending the unlimited supply of gangsters.
Now head back down the interior staircase (the way you ran after Lance) and get back in front of the office. Kill any Forellis there and recover your health with the spawn (there are also health and body armor spawns in a small alcove at the bottom of the interior staircase).
When you walk out of the office, Sonny will show up at the foot of the main staircase with two more guys, where he "reminds" you that your 15 years of incarceration were his to spend, before releasing his goons on you. Shoot Sonny as quickly as possible, as new Forelli soldatos will continue to stream into the main hall. Once Sonny is dead, the mission is over and in the ensuing cutscene, Tommy replaces the now deceased Lance with Ken Rosenberg as his right hand man, explaining that this "could be the beginning of a beautiful business relationship."