I had a tough time with this mission too, but I eventually found an easy way to do it. *What made this mission hard for me was that I just keep driving around the loop where the telephone is. *The Purple Nine vans eventually caught on to me and start ramming me. *This makes it really hard to aim at people on the streets properly.
What I eventually discovered though is to use that road that goes behind your nearby home garage. *This is the road that just kinda ends with a parking lot (Turn right as soon as you leave your home garage). *This road has virtually no traffic on it, but still has plenty of people. *I just drove up and down this road a few times and whacked the 20 Nines with 30 seconds to spare still. *Hope this helps you guys.