You can grab this on the first main story mission that involves combat.
After starting the level, the first enemy will be unarmed. Sneak past him or use combat and you can then scale the wall with your whip.
Here you will find 2 new enemies. Throw the bottle to make one come towards you. Use RB on his club and he will drop it. Pick it up and club him. This will net you the achievement.
Here is a video if it helps