Buy around 500-1000 sniper bullets and go to Vercetti Estate. There will be around 10-20 Vercetti gang members hanging around. Aim for the head and start shooting your own guys. They will not fight back and stand there like a statue. This will raise your wanted level but the police wont come inside the mansion. Even the chopper would just fly around but not shoot you. When the gang stops to spawn enter the mansion and exit again. Repeat until 100 headshots