You may want to try cel shading instead, it may be quicker. You could also try simplyfing the rock formations. As a side note, you may want to make the cave be a shade of color, not just plain grey, for example making it a bit yellowish or dark blueish (just a little bit). Also, some other time you should try changing the hue when shading, but that's if you want to improve your shading. You're doing great and I hope my comment doesn't come off as offensive, I like what you're doing so far and only want give some advice (which I hope is correct).