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I achieved this by firing my gun on the rooftop, getting him to turn around and face his window. Then I dropped down and hoped he would miss. I kept reloading the last save until he did.

Then I climbed back up the tower, and before I walked into his room, I set his gun to the switch under the table.

When I climbed into his room, his gun jammed and I pulled my gun on him. He will intiate dialogue, and you need to press space to move through it. Eventually he will drop his gun and you will get the achievement.

You need to punch him out to finish the level afterward.