During Chapter 7, you will encounter Keepers for the first time. They are the large enemies with a metal box on their head. To get the "I Don't Have Time for This!" trophy, you are only allowed to kill two of them (and those two have to be killed to progress in the mission). The first one you encounter has to be killed to open a door. This will bring you to a gassed area where a lot of Keepers spawn infinitely. You may not kill them. Instead, use the valves to make the gas go away. In the room with the second valve, another Keeper will spawn. This is the last one you are allowed to kill. The third valve is the biggest challenge. It takes some time to turn it off, and one of the Keepers will be hunting you. Lure him to the opposite side of the area, and shoot him with a Freeze Bolt to slow him down. Then, quickly run over to the third valve, and turn it off. Successfully complete the chapter to get the trophy.