Fullscreening the game breaks everything.
The game draws in original aspect ratio but draws the game world in a weird warped way on the rest of the screen, so monsters arent really where they are.
Also the UI disappears so you cant see your skills or health at all.
Also some monsters like the ghost and exploding one will spawn right on you, which only makes matters worse if you take the movement speed upgrade, so you're guaranteed to run into them before they die.