As your first battlers, these are pretty good!
First, did you draw this on paper, scan it in, and color it in photoshop?
the only major problem, i htink i see is the chest plate, which looks a little to round? XD
as for the brain, it needs more shading where the little groves should be
if you make the groves thinner, and in a pink colour, and more random, it wold also look better!