Chapter Select: "Atlas Falls".
You will breach a common room just before the pressure regulator room (see video for exact location). Save all your grenades or flashbangs prior to this common room so you should have x4 of both of them. As soon as you breach the door throw a flashbang and try and pick a few enemies off with your gun. Once the flashbang effects have worn out, throw a grenade at the right side wall and hopefully it'll bounce near the back and take those enemies out too. Rinse and repeat the process until you clear the room. You can do what I did in the video below and just throw a flashbang and then all your grenades. This can also be effective since grenades have a large kill radius There is only about 10-15 enemies in the common room.. If you get shot at all then restart the checkpoint. Here's a handy tip if you are having trouble with this trophy:
Here is just a more easy way to get the "Piece of Cake" golden trophy:
Just breach the door (check the trophy guide for the location video) and when you get thrown away just stay there and do nothing at all! The AI will kill everyone inside for you and you don't have to take the risk to get shot...
Hope this helps.