I did this achievement on the same level as the other answer. I had a problem with consistently keeping enemies out of the designated area though. So I tried a different approach.
Instead of knocking enemies out the areas; I took them on with the unbreakable staff as quickly as possible. It only took like three hits on each enemy to break their structure and do a takedown. Whenever you trigger a takedown it stops the timer. So you don’t lose that much time. You can also surprise one enemy at the beginning of each fight usually, and get an instant takedown. Use your focus bar on the few elites. I think theres a fat dude and a flash kick in the level, but with the staff they fall pretty quickly. I would also use the back-forward-X attack to knock enemies back as well sometimes, to clear room for myself. I got it done with almost 10 seconds left. So there’s plenty of time for some errors. Good luck.