If it's the mission where you have two guys with you and you're killing triad warlords, then the key is to run over as many as you can. When you get a star, drive around until it wears off. When you get two stars, go to the paint shop and get rid of it. When you've run over all you can (I think one might be in their little factory. If so, jack one of their vans and you can get in) there's one in an alley; just get out, press R1, and shoot everything that moves.
If it's the mission where you're supposed to kill "chunky" lee chong (sp) then there's a really fun way to deal with him: Chunky has two getaway cars. Take one and blow it up. Take the other to 8-ball's, plant a bomb in it, park it where you found it. Go in from any direction other than the direction with the car. Go inside until you get the little cutscene, then shoot a couple people and move it back to your car. Chunky will flee, and, well, boom.