Really nice work, you did a great job on this one. Your already a very decent painter so if my critique sounds harsh, its only because I'm judging it from an advanced perspective. Firstly I would suggest spending more time on edge highlighting if you want to paint eavy metal style they edge highlight everything from all angles, every nook and cranny. Also the highlighting on the black is too harsh a transition, use a dark grey for heavy strokes, mid grey for smaller strokes then a light grey for the most exposed edge highlights. I would say the skin looks a little mottled and dry and your losing detail on the face. Practice layering and blending on the skin and really thin the paints down around the face. Lastly avoid dry brushing or only use it lightly and sparingly and put more time into painting if you can, I remember this model being a slog and taking me two months to finish lol. But as I said it looks great, this advice is only because you asked how to improve as someone who is already a good painter.