Get a car (recommended) or bike, then drive fast enough so that you can jump out of it and kill someone at the same time. Drive directly toward the target and jump out. This will not work if you jump out too late. You can also hit police cars by doing this and not get into trouble. You can hit gang members and they will not chase you. However, if you do this to someone the police are chasing, the officer in pursuit will aimlessly run.