Fantastic piece! You took the baddest of the bad. I'm really glad you included a zombie. It's a representation of all the bad ass zombie films out there. Do you ever plan to add to it? I would love to digitalize this one day, with your permission of course.
one thing that I do with shading face is, i don't actually draw noses and stuff with dark outer lines. I take the shading and make implied lines with it. It looks a little better than a dark ass line that's not suppossed to be there lol. And I use my finger for a lot of shading. Just shade a spot and rub it around with your finger. This will give the effect of the shade changing value as you drag it along.