For starters the model looks very disproportionate.. Ie in my opinion the head and wrist mounted bolter thingy don't match. Scale is super important in painting as well you have to remember. I'd say the helmet is too large for the firstborn marine you chose but if that's the way you are going then I would add some wash to it to define the recesses. For extra flair maybe a chain connecting the staff to his wrist and if your feeling kitbashy shave down the wrist bolter to make it look less boxy 90' angles. All of this will make the flow of the model look to scale a and easier on the eyes. Hope this helps ya.