Diamonds Are a Girl's Best Friend/Walkthrough
Drive into the yellow marker at the exchange point, which is an abandoned oil factory. A cutscene shows the deal. The deal becomes botched when Rodislav Bulgarin steals the diamonds and Luis Fernando Lopez and Anthony Prince escape with Gracie, so basically Packie and Niko lose everything.
Gain cover and shoot Bulgarin's men. There are about a dozen, so take your time. Use a grenade only when Packie is far from the detonation point. Once the nearby men have been wiped out, get closer and ambush the remaining ones. Follow Packie to the hallways above ground. There will be a lot of men on the aboveground hallways so use the pipes as cover.
Packie and Niko will split up with Niko taking the "low road" and Packie taking the "high road." The view will focus on three cars that skid on the ground nearby; use a rocket launcher or a few grenades and blow up the cars before the men can spread out and cause trouble.
Meet back up with Packie and corner the two men with the diamonds. Watch as the men drop the diamonds into a moving Biff dump truck below. Packie curses and draws his gun; help Packie kill the men, then slide down the ladder. Drive Packie to his Mom's house in Meadows Park, Dukes. On the way, Packie will lament the failure of their plan.