Diaz is a good method, I prefer to do it without the bullets flying and needing to constantly restart though. Not to say it's hard, just annoying. When I was going for this achievement last night, I was also doing Confiscator at the same time.
The key, is to NOT fire a single bullet or even run over a gang member. Just yank them out and drive for a second (was paranoid that not driving for that second doesn't count towards achievement), get out and repeat. The passenger will fire on you of course, but unless another car/gang member is really close, no one else will.
I've actually jacked one car, drove for my second and got out to another one right away. As if the second car didn't even notice the jack.
Again, don't fire a shot and bring attention to yourself. Kind of hard to jack cars when the enemy gets out as soon as they see you from a distance.