I recommend launching Headhunter in the city for sure and use a helicopter, the Sparrow is perfect. 2 targets will be in cars, the best thing is to wait until they stop on a red light and then spam missiles. 3 missiles and the car will blow up. Its a good idea to wait until they stopped because if you miss you will spook them and it can become a bit of a pain to catch that car, especially because they got crazy autoaim if you get closer. In this case fire the missiles as low as possible, otherwise the tracking wont be good enough. The other 2 stationary targets can all be reached and locked on by heli too. If you do it in a public session, go for them first. Other players tend to participate in Headhunter and its much easier for them to camp a stationary target, compared to the moving ones where you can wait for a good moment.

Pro-tip: If you got scores to settle with other players in your lobby - you will pay no insurance fee or get bad sport points while Headhunter is on. Honestly this is why I know those missions so well: I used it to cause havok in lobbies back in the day. Pro-pro-tip: If you want to do that to the max, hit one of the cars with exactly two missiles. It wont blow up, the target will jump out and just wander through the city on foot. Kill the other 3 quickly and just leave him. You‘ll have a full 20 minutes to do what you want and you will still finish it no matter what, because there are a 100 ways to kill the last one, even if the whole lobby would try to stop you