This page of the guide to Resident Evil Village features a description of the boss fight with Moreau, which takes place near the Reservoir. The following solution will teach you how to prepare for the boss fight, what to do during the fight and how to avoid Moreau's area attacks.
Preparing for the boss fight with Moreau.
Moreau is one of the four lords. He resides in the Reservoir in the southern part of the map. The first direct encounter with Moreau is confusing, as the game will not let you fight him right away. Moreau will reappear in the final part of crossing the swamps. He will then be a large mutant toad.
The boss fight will begin shortly after you jump down to the area in the image above. You will reach it shortly after solving the Sluice Gate Controls drain puzzle.
As you're still crossing the marsh, it's a good idea to collect and create as many mines as possible, as they will be the most useful weapon during the fight with Moreau. After jumping into the fight arena, you can deploy the first 1-2 mines right away. Use more as the fight progresses, preferably when Ethan is not being actively attacked by the boss.
Description of the boss fight with Moreau
Moreau will appear after you get away far enough from spot where you entered the arena. The boss can easily be slowed down with mines which can also be used for further attacks at the same time.
Similarly to the earlier fight with Lady Dimitrescu, look for opportunities to attack the upper part of Moreau's torso which protrudes from his body. This will make him particularly vulnerable to Ethan's attacks.
Use the grenade launcher and shotgun first to deal damage to Moreau. Switch to the sniper rifle and pistol only further down the line. Also, keep checking if you can craft ammunition with your newly collected crafting ingredients.
The boss likes to try and spit on Ethan. You can then quickly move away to a safe distance or hide in side areas.
Moreau can also perform powerful charges that can significantly weaken the main character. When you notice that the monster is trying to charge, you can hide or move away from it.
The battle area contains a lot of loot, which is worth collecting if possible. Look out for more mines first of all, but other loot, e.g. ammo, will also come in handy.
If you don't have any mines in your inventory (and you can't craft any more), you can aim for the red barrels. Fire only when the boss is close enough to a given barrel.
Pay special attention to the moment when the boss climbs to the roof of one of the buildings. This is a sign that Moreau is preparing to bombard the area with poisonous venom.
You must then quickly hide in one of the roofed areas. Don't come out of hiding until Moreau has finished the attack.
Moreau's attack brings one additional impediment - new obstacles similar to the one in the image above will appear in the fight arena . If you are in a hurry (e.g. because you are currently running from the toad), use a shotgun to destroy them quickly. However, it is better to run around the arena and destroy them in advance with the pistol.
You must repeat all the steps described above until the end, that is, weaken the boss with mines and explosive barrels, fire at the "exposed" Moreau, run a lot to avoid the toad's usual attacks and hide under the canopy when the boss bombards the area.
Defeating Moreau will reward you with the Fish Out of Water trophy and the very valuable Crystal Moreau Treasure. You will also be able to venture further into the mine.