Silence, Fool!
Kill a guard ringing a bell.

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This can be done at any time in the game, free roam or during a mission. Because alerting the guards during a mission is generally a bad thing, aim to do this during free roam. One location that is not overly difficult is Tortuga (882, 370). Travel there and locate the only viewpoint on the island. It is on a building that has an alarm bell just to the east of it. Sneak in and get close to that bell, then let a guard detect you. He should run up to the alarm bell and start ringing it, so step up behind him and press E to assassinate him. You can then jump in the water to evade any pursuit or battle it out with any nearby soldiers.

Note that you need to let him actually ring the bell at least once before you assassinate the soldier for the achievement to unlock. If he has yet to grasp the rope, it won't count. You also need to kill him in one blow as otherwise he will stop ringing the bell and defend himself or attack you, and it won't count for the achievement.