Bring On The Blood Rain

It is the 6th and final boss and is found at the bottom of the Fleshsore Layer. Warning; this boss is a lot harder with a melee build but there is a solution, get a concussive weapon.

It has two different attacks that will kill you in one hit. The first are the expanding eyes that spawn randomly around the room, and the second one is the eye attached to it that moves horizontally across the room. To avoid the former you'll have to hide hugging one of the walls on any side of the screen. In order to deal damage to this boss, you have to destroy the four eyes around It. Once all eyes are gone his big eye will open up allowing you to damage him.

You can narrow the space where the eye can move horizontally by building walls on the sides and leaving a space at the top or bottom where you can shoot. This way, the boss' movements will be more predictable and you can hide behind those walls and leave an opening where you can shoot safely from.

If you are playing as a melee character your best bet is to either use a concussive weapon to shoot from behind the walls or bring lots of healing potions.