Story-related and unmissable. Mergo's Wet Nurse is the final boss of the Mergo's Loft area of the game, and this boss fight is triggered by approaching the cradle at the top of the big circular arena at the top of the area.
Mergo attacks swiftly and frequently with her many knives, but fortunately she has a narrow region
directly behind her where the majority of her attacks will not hit here. Stay in this region right behind here (not at her side!) and you will be OK for most of the fight – you can even attack while she is attacking most of the time.
The Wet Nurse becomes most dangerous when she lets off a purple aura that turns the entire arena dark. The darkness lasts quite a while (about a 45 seconds or so), and Mergo can summon a clone of herself to attack you during this portion of the fight. Do not try to attack Mergo much while this is going on; instead, just focus on staying alive and keeping your health topped off. Once the darkness fades away, resume attacking her from behind. See the following video guide: