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This achievement is missable and is earned during Mission 16, Stopping Marchenko.

First and foremost, you must pick up the Kill Switch before progressing too far into this mission and initiating the fight against Marchenko. After the cutscene with Miller at the end of Mission 15, leave the kitchen the way you came in, only this time, take the corridor on the right-hand side. You will reach a security room with one patrolling enemy. Sneak past him or knock him out; once you're in the room, search underneath the desk on the left-hand side for a weapons briefcase. This briefcase contains the Kill Switch!

With this item in your possession, you can proceed to Marchenko's position. You will be thrown into dialogue as soon as you open the doors to the Exhibition Hall. Marchenko will taunt you and you'll have the opportunity to reveal yourself or use the Kill Switch. Pick the "Kill Switch" option before the timer expires and the achievement will unlock shortly after.