When you finally reach the meeting room, there will be two glasses on the table in the middle of the room. Either destroy them both for a nonlethal solution, or switch the glasses so the Overseer poisons himself. If you chose the first option, you'll need to follow the men down to the basement, where the Overseer will show Captain Curnow a painting, and pull out a sword to kill him while his back is turned. That's your queue to move in and take out the Overseer, so Cpt Curnow realizes you're saving him, otherwise he will call the guards. The achievement will pop once Curnow walks off the property by himself.