Mergo's Wet Nurse is the final boss of the Nightmare of Mensis. She has devastating attacks using sweeping blades, as well as a number of very fast long range lunge attacks. Her attacks are mostly frontal however, and therefore if you can get in behind her you can damage her in relative safety.

The largest challenge of this fight is a phase she periodically triggers, where heavy fog sets into the arena and she summons a clone. You have limited vision during these phases, and although you can damage her, your efforts should be focused entirely on evasion and staying alive. The long range lunge attacks of both her and her clone mean that you are constantly under threat, and the damage potential from these attacks mean that you can quickly be cut down if you remain stationary for even a moment.

These phase's last around half a minute, and the rest of the fight is manageable so long as you can survive them.
 

How to Beat Mergo's Wet Nurse

How to find the boss

You must have defeated Micolash in order to be able to reach this boss. A cutscene will be triggered, showing you that the bridge at the top of the building is now in place. Cross this bridge, light the lamp, then head up the stairs. The nurse isn't hard to find, just ignore the elevator you need to use in order to find the Blood Rock but take the stairs in the opposite direction instead. You will encounter some Shadows of Yharnam which you should take out one by one if possible. Continue down the path and you should come to a place with a couple of boars. The first one will be alone and shouldn't be too difficult, especially since you can take half of its health off with a surprise attack. The other two boars might charge at you together, so be careful here and focus on one boar to get rid of the double threat as soon as possible. A little way farther down the path there will be a room to your right. In this room there is an elevator. Use it to get upstairs; you should be in a room full of Mico-like figures sitting in chairs - that's it, you are right in front of the boss area.

Preparation

Depending on your level and on your damage output this fight might be rather long and some of the attacks of the boss are rather devastating so make sure to bring 20 blood vials. If you consumed too many of them while trying to find the boss it might be wise to stock up on them before attempting the fight.
 

Boss Fight Strategy

Mergo's Wet Nurse wields about a trillion scythes, so whatever you do, you don't want to confront her face to face. The trick is to get directly behind her. Use to lock on and dodge her attacks to get behind her. You will soon find out that some of her attacks are better suited for that than others. Once you are behind her it's not too complicated to stay there though, just watch her closely and keep dodging sideways. You are relatively safe behind her back, especially while she is busy slashing thin air to counter your hits - you can even use your attack in this case to deal some massive damage.
Unless you are really quick she will summon a clone after some time. As it is absolutely impossible to stay behind two backs at the same time it's highly recommended to gtfo and avoid combat at all costs. Keep running, dodging and rolling until the purple mist vanishes and the Wet Nurse is alone again. Now you can resume the strategy exlained above.
 

Rewards

72,000 Blood Echos
 

Great One: Mergo's Wet Nurse Video Guide

Video by PowerPyx.