I think your biggest problem here is that your boss doesn't have a defined shape. It's a blob with arms and legs, and by looking at the sprite or the big image you made, I can only see all the limbs share the same plane and are all the same thickness as his body.
I made a quick edit to illustrate some points:
- Each limb is either in the front or the back. This is done via lighting and by making them overlap, creating depth
- The face is more centered, so we have an easier time seeing all facial features as part of a whole. Lighting in the eyes and cheeks add some personality
- Used lighting to represent shapes better. Imagine each part of him as a single shape and how the light would hit it from an angle. The lighting on the arms help create a cylindrical shape. The little glow in the legs represents how the knee would be hit by the light. Each spike is illuminated on its own, same with the spherical hands
Now that you have a defined shape, once you make a bigger version you should be able to increase the details, such as: shading the tongue, shading each of the spikes in his head, adding color to the iris, adding folds to the gloves, etc, all according to your art style of course
Put some time into fully realizing the character. You made a small sprite and then just upscaled it, where in real life whenever we look at something closer we see more detail
Hopefully this helps ^^