One of the easier ways to do this is to go through the game entirely, reset the gangs, then go through the Los Muertos gangs. You should have no problem taking down 200 gang members without dying.
If you want to attempt both this achievement and "Roadkill King" at the same time, follow this strategy. Start at the Agency Keep and pick the Super-car. At maximum driving ability this car is capable of shooting enemies (use ). Don't use the vehicle's firearms to kill your enemies, just to attract them out of hiding to run them over.
Otherwise, just be very careful and try to pick off gang members from a distance. It's best to have use a sniper rifle and the Firefly and stick to reigning fire down on gang members from the rooftops.