During the first mission "Justice For All", you will be tasked with entering the security center in as stealthy a manner as possible. You can get through most of this section in stealth by using the ghost generator, but you will have to flush out the captain in the actual security center with a gas grenade and kill him silently. To do this, head to the far side of the building and crouch under a pipe. From there, climb a nearby pipe and turn the valve a few times. However, turning the valve will usually cause three Helghast to begin patrolling, and if you continue using the valve, they will be alerted. So, turn the valve in short segments and kill any soldiers that get near.

Once you finish using the valve, throw a gas grenade into the security center and then kill him with a silenced weapon or interrogation. Provided you have cleared this whole area without sounding an alarm, you will get the trophy here after your partner accesses the computer. You will be required to do this as part of the covert contract and to obtain a piece of intel.

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Credit to PowerPyx for the video