This is an easy achievement but can be made really difficult if the host is shelfish or spiteful. Obviously, this must be on one of the DLC maps. This is made difficult if the host is losing/sore loser and closes the session early. Some times they will wait and close it right before you get in the escape vehicle. If not boosting, the best way to get this is try to get host yourself and start a fragile alliance match with the min. 4 players. If you are hosting, you can pick he level. Highrise has a lot of money so fighting for money with other players will not be a problem. Just grab your 100k and back off. Most players will try to kill you. The trick is to let them do the work. Dont get too close to them as most of them will try to kill you. They will kill the NPCs for you. The hard part is if they die and you have to kill the NPCs yourself. This is the best way to do it. Alternatively, you can boost with 3 other players. Which will make things a lot easier of course.