I did this during Combat Challenge 2.0, which is located in Midtown after completing Combat Challenge 1.0. There are 20 enemies here, the strongest of which will use crowbars and handguns, so they aren't much of a threat. You want to try and defeat them all with one combo.
The first few enemies will only attack you close range so you can stick to just using melee and web-strike on them to build up your combo. I'd recommend mostly sticking to just melee and web-strike on the remaining enemies to give you time to reach 100x, though I was able to use takedowns and venom punches on a few enemies and got the trophy before all 20 enemies were defeated.
The trophy will pop as soon as you reach 100x on your combo. If you get hit or break your combo prior to 100x, you'll want to restart. You can also retry Combat Challenge 2.0 after completing it so you don't need to worry about earning the trophy before completing the challenge.