So with shading a game sprite you want the light to come from the top, that way you can mirror the sprite and not make it look like the light source jus switched sides. I think what you have now works as well, it looks like the light is coming from the front (front like into the front of the screen, not front of the sprite), which doesn't shift the light source either when the sprite is mirrored.
As for your shading I think you did a great job, the only thing that should be changed imo is the highlight, it shrinks too much, Its good that you make it change, but this is too much.
And as a giveaway:
Slimes should be slimy!!!