Pretty straight-forward... there are nine missions, each has two bonus objectives. Complete them all and unlock this achievement and a special blunderbuss!
*** Spoiler - click to reveal ***Most of the objectives are pretty easy, but two gave me trouble. In the last mission, you have to kill the boss using his weapon. Took me a minute to figure out: to disarm him, unequip your weapon (press down on the d-pad to use your hands), then we he attacks hit B to counter and then quickly A to steal his weapon.
The only other one was mission 6 or 7, where you have to do all five investigations without entering combat. There is a checkpoint after each of the first three, then the last two you have to do without a checkpoint. So, try to do the harder ones first! For me those were the eavesdropping and freeing the slaves from the jail.
Good luck!