Instructions
Get a fresh Kevlar vest before you talk to Diaz for this mission. You don't get a chance to go anywhere once the cinematic is done (you get straight into the helicopter).
Try out the M-60. It's unlike other guns and you've got unlimited ammo, so try taking a few shots at passing cars, boats, whatever you can hit.
Waste every ganger you see. Lance will stay in an area until all the gangers are down, so take 'em out! Look for the smoke puffs from their guns and be quick. The helicopter won't survive forever.
Head inside once Lance sets you down and clear out the house.
Grab a machine pistol off one of the gangers if you're low on ammo. They all drop the same one, so ammo shouldn't be a problem.
Head up the stairs when you get inside, then to the left.
Head up the second stairs and onto the roof. Grab the briefcase and you're done!