I did this a different way than the other guide. Once you have access to the Roxxon lab bases and remote mines, go to the one base in Hell’s Kitchen with the 60 combo bonus objective. Once inside, there will be a lone guard fixing a generator. Take him out with a remote mine. Pause and abandon mission. It will put you right back at the starting point outside. Jump down and restart mission and do it again. Rinse and repeat. You also don’t have to worry about running out of mines because they reset after each time you abandon mission. Works just as well for ceiling takedowns too. Took a few minutes to get it this way. Enjoy!