maybe use something dark and close to your darkest color in the pallette... i wouldn't use 0,0,0 black, for sure. this still looks VERY good, though. good job!

If you were to make the dark color for that sprite not black (a more dark, desaturated blue) than you could keep the outline I think.

I'm limiting myself to 6 colors per sprite, 4+black and white. I'm thinking my solution here would be to just not have an outline on the wave...

the edits helped, but the water one still needs some help on the jusmps to black too quickly issue, IMO

I made a few edits to it per your critique, I hope it looks better now.

the outline is inconsistent.

first monster has full black outline, the last two have outlines that jump to black a bit too quickly. last guy has just the horns in black, so it flattens the image, as if the horns and body where different layers. some highlights on the arm may help maybe. tornado looks kinda like a cornucopia.

That actually wasn't the fourth monster. :C They just happened to have similar colors.

What can I do to imporve the last one?

 the last one seems to be of worse quality :( I love the other 3 though.  I saw your wip of this in the forum, and much prefer your other monster you had for the 4th