Wheelman

I found the best car for this, bizarrely, was the Astra. It was nippy, and packed a punch. You may have to give these a few shots or the S rating, learn the route, and just smash them into oblivion. Try not to get wedged against the target, as you can't damage them, and don't settle for hitting them once, once you make contact, slam the analog stick towards them as often as possible, you'll find if you hit them all the way across the road and slam them into a wall you can kill them really quickly. If you're struggling to get alongside them because they're too quick, boost and use the front melee and this will damage them and slow them down.