This achievement is story-based and cannot be missed. After you kill the Mistress, the path to Venice will be opened and the achievement is unlocked.

During the boss fight, use a hammer to destroy the five pillars holding a structure from which she is throwing the shurikens. When the structure is destroyed, she will also fall down and you can use any of your weapons to kill her. I found my hammer very efficient, but she is not too difficult with whatever weapon you use.

Once you kill her, there is a second fight where you have to kill her in her demon form in the Twilight World. Just roll from her tentacles and when the tentacles roll in, use your Moonblade to kill them. If you do not go near to her, the tentacle attacks can not even reach you, but the tentacles still get close and can then be attacked. Once you got rid of the five tentacles, go for her head and roll to avoid her poison attacks.