I did this a different way than the other guide. Once you have access to the Roxxon lab bases, go to the one in Hell’s Kitchen with the 60 combo bonus objective. Once inside, there will be a lone guard fixing a generator. Web zip to the ceiling and do a takedown. 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. Works just as well for wall takedowns too. Took about 5 minutes to get both this way. Enjoy!